Guide to 5 Star No Bonus runs

What is a 5 Star No Bonus run (5SNB)?
It's exactly what it says on the tin. A run of the game on 5 Star difficulty where no second game bonuses are taken, although an exception is sometimes made to allow you to enter the Kill The Monkey Route (KTM).

Why should I play 5SNB?
Sadly my honest answer is that you shouldn't. All that you're really going to get from it is bragging rights and it's a sad fact that if you're playing well then it doesn't take long for a 5SNB run to become indistinguishable from a normal 5 Star run. If you want to do something challenging then I'd suggest that you either try a Lowest Turn Count run like this one or a points run (this is the best English one that I know of, although even its own author admits that it's a poor one and both the Japanese and the Chinese [1 and 2] have done much better).

What should I know before reading this guide?
Above all else, you need to know that this guide has been written with the following assumptions:

Finally, for reference you should know that the first 50 points of SAT are intended to be spent as follows. All of these choices will be justified within the guide.
 * 1) You have previously beat the game on 5 Star difficulty and have a good understanding of how to play both the game and the route that you wish to beat on said difficulty. Consequences of this assumption include the fact that I'm not going to be telling you what to do if you happen to capture a fantastic generic, when/how to upgrade most of your units, or why you should scout before almost every battle.
 * 2) No savescumming will be assumed in this guide. After all, 5SNB is a challenge run - why would you cheat it?
 * 3) You are not doing the KTM route. While you could probably still use this guide for a 5SNB KTM run, its early game is just too different from the other routes for me to bother adding special notes for it at every point in the guide.
 * 4) The reader recognises that there exists situations where this guide should not be followed and is willing to adapt to them as need be. This assumption is mostly here just to cover those runs where the RNG has given you a very bad time.
 * 10 SAT - Rance Attack


 * 20 SAT - Recruit Dousan


 * 30 SAT - Call for Rizna


 * 40 SAT - Call for Uruza


 * 50 SAT - Third Action Fan

What route should I enter?
Given that this is a challenge run I suppose that it's best for me to rank the routes by difficulty and let you pick one of them (see below, hardest first), although, for reasons that may soon become obvious, I personally prefer the True Route.
 * 1) Demon King Route entered via the True Route after the Battle Permits have been lost - This route is obviously harder than the True Route and is therefore the hardest route in the game. Personally I'd advice against entering it, it's so hard that the only fair way to beat it is to resort to boring strategies like abusing Initial Time Advance (see walkthrough).
 * 2) True Route - Simply listing the victory conditions for this route and comparing them to that of the Isoroku Route will show that this route is harder than the Isoroku Route and therefore this route gets placed above it. Furthermore, even if it weren't for the difference in victory conditions the simple fact that you'll not have your Battle Permits in the end game of the True Route would, alone, be enough to earn this route the #2 spot.
 * 3) *For the sake of full disclosure I should mention that I know of two arguments that have been used to claim that the Isoroku Route is harder than the True Route. The first argument claims that because you can't get Seigan in the Isoroku Route, the Isoroku Route is harder than the True Route. Needless to say, this is easily rebutted by pointing out that the unrestricted Battle Permits of the non-True routes are better than Seigan is. As for the second argument, that focuses on the fact that in the True Route you can exploit the temple event in order to gain unlimited free time in which you can upgrade your units. However, in my opinion this argument can rebutted by appealing to the Isoroku Route's unrestrictred Battle Permits. Specfically, I'd argue that in most realistic circumstances, unrestricted Battle Permits can easily be used to stall for an infinite number of turns, meaning that the point of the True Route having an unlimited time exploit is moot. Regardless, if you like either of these arguments then by all means do the Isoroku Route instead of the True Route and if you feel like it then I suppose that you can also tag on the Demon King Route near the end of the Isoroku Route. After all, this is supposed to be a challenge run.
 * 4) Demon King Route entered with Battle Permits (i.e. either via the Isorku Route or prior to losing the Battle Permits in the True Route) - The fact that you have the Battle Permits for this entire route is enough to make this route easier than the True Route and unlike what we have in the #1 spot any serious problems (e.g. Miki) can be ignored via competent use of scouting,
 * 5) Isoroku Route - I've already shown that this is easier than True and it's obvious that the Demon King Route entered with Battle Permits is harder than this, hence this gets this spot.
 * 6) Kenshin Route - This is easier than the Isorku Route because firstly it requires you to conquer less land and fight the Demon Army less than the Isoroku Route does (e.g. the Kenshin Route doesn't require you to conquer Senkan Nagato, but the Isoroku Route does) and secondly because a skilled player can beat the bulk of this route in a matter of only a few turns.
 * 7) Ran Route - This route almost plays itself. Placing it here requires no justification.
 * 8) KTM - The big problem for this route is that, by design, there's no time pressure. I do however fully admit that if you don't exploit the lack of time pressure then KTM might move up a few ranks, but the lack of time pressure is pretty much the entire point of the KTM Route, so I'm not sure why you would do that. As an aside, I'll mention that I'm strongly against 5SNBing this route. Even if you ignore the fact that you have to take a bonus to enter this route and therefore it's not really a 5SNB run, the big problem is that you'll never be given the Battle Permits in KTM and because of that the game really starts to drag. If you're going to 5SNB this route then expect to get bored pretty fast.

What is the layout of this guide?
Generally speaking I'll tell you what to do and then there'll be a subsection explaining why I'm telling you to do it followed by another subsection of notes which cover anything that I think is worth knowing. This is generally done so that you can read the whole section in one sitting and then just refer back to the overall plan for that part of the game whenever you want to sit down and actually do it.

The Big Plan
Your route through the game is quite simple. Hara+Ashikaga->Miko Institute->Uesugi->tie up loose ends->enter route->vassal Tenshi Sect->beat route

Why this route? I'll break it down.
 * 1) Hara + Ashikaga - Past turn 2 you don't really have much else to do other than go to war with either Hara or Ashikaga and as far as I can see there's no good reason to delay these two wars or to war anyone else - Iga is unrewarding and Tokugawa isn't worth the trouble at this point.
 * 2) Miko Institute - This comes next because it's simply the best choice. Natori is arguably the best unit in the game and taking her house gives you not only a chance to grab some of the game's best semi-generics, but it also gives you the path to the Kenshi+Uruza+Dousan+Natori combo which few parts of the game have even as much of a chance of withstanding. Aside for this, you just don't get anything nearly as good from your other options. For the sake of completion, I'll go into detail on why you shouldn't go to war with the other houses we have available at this point:
 * 3) *Iga, Akashi, and the Tenshi Sect can be ruled out as being detrimental, the first two break a gourd and give you virtually nothing for it and as for the Tenshi Sect, I'll explain that later.
 * 4) *Takeda and Tokugawa - Particularly in comparison to the Miko Institute they just aren't worth the effort for what they give you in return, and that's assuming that you're even capable of beating them at this point. The fact that conquering Takeda triggers a war with Houjou isn't very nice either.
 * 5) *Tanegashima is a poor choice, the power of its muskets make it risky for the lives of your commanders and in return it only really offers you Yuzumi, who you don't even have the Worker Bee for yet. Using this as a path to Mouri doesn't strike me as a good idea either. Kiku's Assassinate, the war with Takuga, the multiple tough castles, and even Mouri himself are going to make this a very hard path to go down. To make matters even worse the fact that you can't cheat the boss fight against Mouri with Alice's Mansion could even mean that you simply cannot defeat the Mouri House. As for the rewards, assuming that they outweigh your losses, what are you even going to use them for? If it's beating castles, then you should've just took down the Mikos and got Natori and if it's normal fights then the Battle Permits (which you've probably already got by this point) will make them redundant until much later on meaning that you would've been better off avoiding Mouri until a point later in the game where Mouri will be easier to fight. Long story short, Tanegashima is a risk that's not worth taking, and going to Mouri from Tanegashima is going to be a pointless nightmare.
 * 6) *Asai-Askura offers you one big thing that the Miko Institute can't - NP. Well, it does on paper at least. Too bad that it doesn't really stand up all that well to closer examination. On top of being nearly useless units, Askura Yoshikage and Yukihime take a total of 4 NP from you just to recruit which already takes up 80% of the NP that was on offer from conquering their house. The fact that you need a lot of affection items to get any use out of them doesn't help either - you need 3 clears worth to prevent Yukihime causing a war, scare off her unit for 5 SAT, and get the NP bonus from her dad. This would be nice if Yoshikage's NP bonus was substantial, but this early on in the game it's nothing of note. Overall they're best left until later.
 * 7) Uesugi  - This should be a no-brainer. None of the already mentioned options have become much more appealing and you'd be a fool to turn down an opportunity to get your hands on the Kenshin+Uruza+Dousan+Natori combo, particularly given how easy Uesugi should be to beat.
 * 8) Tie up loose ends - This one is a bit weird. The biggest thing that you're going to be doing after Uesugi is just wait for your Battle Permits to become available. With the Kenshin+Uruza+Dousan+Natori combo you've already got a team that can beat just about anything the game has to offer so all that's left to do is improve the weaker parts of your army and in my eyes the only obvious things that your army is likely to be lacking are good Footsoldiers, so grabbing Senhime from Tokugawa should be a pretty sensible thing to do and once you've got that you might as well grab Imagawa and its optional 5 SAT. After Imagawa however, there's too much variation between individual playthroughs for me to offer any real advice. Just conquer whoever you think is a good choice to conquer and once you've got your Battle Permits up and running then all that's left to do is enter the route that you want to enter.
 * 9) Enter route - I could give advice here, but the problems are two fold. Firstly, there's too much variation from playthrough to playthrough for me to give much advice and secondly there's no need for me to give any advice any more. By this point in the game you're pretty much back to the difficulty of a 5 Star run with bonuses and therefore you should already be in a position to know what you need to do.
 * 10) Vassal Tenshi Sect - I'll explain this later.
 * 11) Beat route - By this point you're definitely back at the level of an normal run and therefore you no longer need me.

What to do

 * Turn 1 - Use your first Fan on 'Dango princess: Kouhime' and use your second to recruit one of the units who was made available by that. Don't waste money healing from the first fight, it'll happen naturally.
 * Turn 2 - Recruit the other two units who were made available by the 'Dango princess: Kouhime' event.
 * Turn 3 - Do the 'Prison (Capture enemy commanders)' event and declare war on the Ashikaga House (Hachisuka Muneya and Akechi Mitsuhide are the least wasteful units to use to declare war), and buy troops for Rance. Defend against any attacks from Ashikaga.
 * Turn 5+ - Recruit Yamamoto Isoroku and when possible, give her Whirlwind Shot.
 * This has a few parts to it:
 * Part A (strictly unnecessary, but very helpful) – Trigger the event where you auto-lose to her. The best way to do this is to just send Rance's unit out alone to fight any battle against the Ashikaga House after Isoroku has been introduced. Note however that despite the fact that there's no need to scout in order to trigger this event (it'll trigger for any battle against the Ashikaga House which includes Rance), you might want to scout anyway, just in case you happen to come up against a poorly defended Isoroku, making for an easy part B.
 * Part B – In a battle which you win, reduce her troop count to 0, triggering the event where she is fleeing from battle, followed by the event where she is scolded by Ashikaga Choushin (see the notes for details)
 * Part C – Capture and recruit her (see the notes for details, but it is of particular importance to note that once you've seen the scolding event, Isoroku can be insta-killed and is under most circumstances guaranteed to be captured if you annihilate her unit in battle and is therefore easy to obtain post scolding {just solo her with Suzume})
 * Part D – When possible, give her Whirlwind Shot.


 * Turn 6 (meeting Suzume) - Use the 10 SAT bonus for Rance Attack.
 * Turn 6+:
 * Conquer the Mamushi Oil Field
 * Vassal the Hara House (this can be done any time after taking two of their territories, even after said territories have been lost. If Owari isn't at risk then blitzing the Hara House is often a good way of achieving this)
 * Conquer Ashikaga
 * Recruit the Mino Three, followed by Dousan (use your 20 SAT Bonus).

Why you do it

 * Dango princess and the recruitment must come first, you've simply got nothing else worth doing on turn 1 and as for the recruitment at turn 2, those units are going to be pretty helpful in the next few turns, so why not get them now?
 * NB: Sill's 5 SAT scene will happen as a start of turn event, it's not worth wasting a fan on.


 * Leaving the prison event until turn 3 is optimal because it means you'll have more of your units available in case Ashikaga attacks (which is good, because killing their troops now means not dealing with them when you're attacking them next turn). In other runs it would be optimal to not do this event at all since capturing someone automatically (e.g. by conquering the Hara House) will automatically open the prison, however in this run we want to get Isoroku before we'll get a chance to do that (explained later) so we need to open the prison manually. As for why we buy troops for Rance, aside from the obvious reason of increasing the survivability of a unit who must not die, it's because you're going to need them later on for capturing Isoroku.


 * Declaring war on Ashikaga (and not Hara) at turn 3 is optimal for reasons which were discovered by reading the game's code. To put it simply, one of the consequences of this being a run on 5 Star difficulty is that defeating the Hara House, whether that be by vassaling or by conquest, significantly increases the strength of the Ashikaga House. This leads us to a strategy which can be used to minimise the difficulty of both the Ashikaga and Hara houses. Specifically, we capture Isoroku and conquer the Mamushi Oil Field, vassal Hara, and then conquer what's left of Ashikaga. Counterintuitively, the key to this is actually Isoroku (explained below). Speaking of Isoroku, she's also why you declare war on Ashikaga manually rather than doing it by refusing to pay them (which incidentally, isn't always an option at the end of turn 3, Ashikaga sometimes gets a different event), doing it manually will make her be introduced one turn faster than refusing to pay them, giving you one more turn of opportunity to capture her than what you would otherwise have (this extra turn also comes when Ashikaga are at their weakest, which is nice).
 * NB: With a little bit of luck there's an alternative tactic that you can use at this point (known as '5-Oil'), however without reloading it's impossible to know ahead of time whether or not you're going to get this luck, so this tactic can be seen as a little bit of a risk. The basic idea is that you play the first two and a half turns as you normally would but rather than manually declaring war on Ashikaga you instead do something else with that fan and then hope that when you end the turn you get the event where Ashikaga demands money from you. You then choose to not pay the money and hence have Ashikaga declare war on you with Isoroku's introduction delayed until the end of turn 5 which (with some luck mostly relating to where Ashikaga attacks and what with) will give you a good shot at conquering the Mamushi Oil Field during turn 5. Needless to say, if this works then you're in a great position (if you have enough NP you can even get Dousan as your first SAT bonus), however if it goes wrong then you might be in for a bad time. Sometimes you'll recover from it easily and other times Ashikaga and Hara will respond to your blitz tactics with their own blitz tactics and you won't be far off from a Game Over.
 * When doing this make sure that you have a non-zero amount of gold, failure to do so will prevent the event where the money is requested from triggering.
 * If you're wondering why this is focused around getting the declaration of war from Ashikaga at the end of turn 3 rather than waiting another turn and seeing if it happens on turn 4, the simple answer is at the end of turn 3 there seems to a 50/50 chance of getting either the desired event or the first event with Yamanaka Kojika, while after the end of turn 3 it is often the case that other events will take priority over it (e.g. Tokugawa's intro), giving you as good as no chance of seeing this event until much later on.


 * Recruiting Isoroku is such a high priority because you simply can't make much progress against Ashikaga while she's fighting for them, she's their strongest unit by far and her presence alone is often what's going to stop you from conquering their castles (if she's in Ashikaga's army then she's guaranteed to appear at their castle battles). As for why you want to arguably waste a fan letting her automatically defeat Rance, that's because Rance is your best unit at this point and you're needlessly handicapping yourself if you choose to fight Ashikaga without him. These two subjects are actually quite closely related - Isoroku is such a powerful unit that I'd advise not attending any battle against her unless she's either very badly defended or if you've got Rance, preferable with Rance Attack, who is guarded by a good Footsolider. Not having one of these conditions satisfied will mean that you can't make up for the deaths that Isoroku will cause, losing you the battle (or if Rance dies, the game) and any chance of getting her to 0 troops.


 * You rush giving Isoroku Whirlwind Shot because it makes Kyo's castle much easier and it will come in very handy against the Miko Institute later on. If you're worried about the lost SAT then don't be, over the course of the game you'll easily get her back up to 7 Normal.


 * Rance Attack is chosen as the SAT Bonus and taken so quickly simply because it's so helpful at this point, it makes Rance a much better unit and that's a tough fact to ignore.


 * To completely explain why the Oil Field is optimal to conquer first, I need to go into some detail. What appears at the Ashikaga castles depends on whether or not you've captured Isoroku and whether or not it's their last castle. If you've not captured Isoroku, then the Oil Field has 3 warrior units in the front row, with Isoroku and up to two random units in the back. If you have captured her, then she is replaced by another random unit in the back row and one of the front row's Warrior units is replaced by an Archer. If the Oil Field is their last castle, Chousin will also be in the back row. As for Kyo, the front row is always a Footsoldier, a Ninja, and a Warrior while the back row is up to 3 random units, replaced by Isoroku and/or Chousin depending on the same factors as above. Long story short, if you've not captured Isoroku then the Oil Field is the hardest of the two, at this point in the game you just can't handle the offencive force of 3 warriors and Isoroku's bow. Once you have captured her however, the Oil Field is probably the easiest - the totally exposed Archer will fail quite quickly, and with Shibata to eat hits the two Warriors shouldn't be a problem (unlike Kyo's Foot solider, who can tank pretty well if you're unlucky). So on difficulty alone you should be targeting the Oil Field, but there's more reasons than that. Having the Oil Field means that you can make progress towards getting the Mino Three and Dousan and it also gives you access to the SAT giving events which are needed to go to war with the Miko Institute, and it also gives 1 more NP to you than Kyo, while the only interesting thing that Kyo offers you are the two Kuge sisters and a SAT giving dungeon, all of which aren't anything special when compared to the Mino Three, Dousan, and the Miko Institute's events.


 * As for why Hara is vassaled, and why it is vassaled prior to conquering Ashikaga, this can all be boiled down to the same reason. You've probably lost a lot of Owari's territory to Hara and vassaling them is the easiest way to get you back into the safe zone of having Owari completely under your control and only having Ashikaga to worry about. For the sake of completion I should say that there's more reasons to it than this (e.g. Hara's castle is very hard to get through and isn't all that rewarding to beat) but most of them aren't worth mentioning.


 * As for why Ashikaga is conquered rather than vassaled, the simplest explanation is that unless things have gone horribly wrong (see the notes for this section), you'll gain nothing of value from vassaling Ashikaga.


 * As for the Mino Three and Dousan, Dousan in particular will be needed for your war against Miko Institute. I can't stress enough how good she is. She's a Monk with great stats who is usually recruited with a high troop count and also comes with the game-breaking Convert Action. As for the Mino Three, I'll discuss them further in the Miko Institute's section.

The Miko Institute
Before starting the war, you should take a break, both in-game and out. This is the last major hurdle before you'll be back to the familiar ground that is the difficulty of a 5 Star run WITH bonuses, so you really don't want to trip up here. Here are some pointers for before you start the war:
 * 1) Check your items, many people forget to give Rance their textbooks or to give out affection items. Now is the time to correct those mistakes.
 * 2) Work on some events. How many events you do here mostly depends on what the current turn is and how many free Fans you had while fighting Ashikaga, although your absolute deadline for declaring war on the Mikos should be during the turn where Rance meets Apostle Rengoku for the first time, any later than this and you risk Iga declaring war on you (NB: this makes doing the events which are required to be able to declare war on the Miko Institute a high priority). The important thing however is that you don't over do it, if you do too many events now then you'll have nothing to do later on when you've got a spare Fan. Generally speaking it's probably safe if I just advise you to use your own best judgement however given that Dousan is such an important unit for this part and has probably started out on well over 500 troops, feel free to dump a bunch of money in getting her into the 900s. You could even do a couple of levies just to feed her troop count and give Rance some more textbooks, the fact that Dousan is a Monk should make this pretty painless in the long run.
 * 3) Have a think about what's best for your army. In particular, you'll have to consider which (if any) of the Mino Three that you're going to fire. In my experience the Diviner is a very useful unit and should be kept pretty much no matter what, however the same can't be said of the Warrior and Tactician. If you can replace them with better units then by all means do, I almost always fire the Warrior immediately after getting Dousan and keep the Tactician for whenever I need him (he's a pretty good archer and it's hard to say no to the free buffs) and for a Battle Permit later on. Another big issue is your Prison, the short and simple advice is just for you to grab anyone that you need to hold back the Tenshi Sect while you fight the Miko Institute, particularly Footsoldiers. As for how you're going to be using your army, your front row for any tough battles should now be Dousan, Shibata, and Rance, and the back row should include Suzume and Isoroku, along with another unit who's appropriate for the enemies you're facing. The Kuge sisters are also worth a look.

As for the war itself, there's no non-obvious step by step instructions for beating the Mikos, so all I can give you is a set of notes:
 * Over the course of warring and conquering the Miko Institute you're probably going to gain at least 20 SAT so it's worth noting here that your 30 SAT and 40 SAT should go on getting Rizna and Uruza (Rizna should be got sooner rather than later, you don't want to have to wait extra long for Uruza), and your 50 SAT should go on the Action Fan. As for why these are the optimal choices, it's just simply the case that none of the other choices can even compete with them. Uruza and the 3rd Fan are just too good.


 * If you're having any NP problems, consider conquering Mikan. It's two easy battles and it gives the Tenshi Sect something other than Kyo to attack. Speaking of Kyo, until you've beat the Mikos, that's the easiest place you've got for upgrading the NP of, so that's another option to deal with any NP issues.


 * As for how you should be fighting, this is the point in the game where what you've learned over multiple playthroughs becomes more and more useful. Use your good units against the Miko Institute and don't hold back against them. Treat every fight as if it was a castle battle and unless you've somehow got a bunch of really good units, only attack once per turn. Use your bad units for scouting and winning defensive battles.


 * While turn 50 is your deadline for Takeda's gourd popping, I wouldn't be too worried about a long fight against the Mikos. Their generics are good enough to replace any non-serious losses, and some of their semi-generics are awesome.


 * Here's some general tips for the individual battles with the Miko Institute:
 * Sheer persistence, even for the castle, just retry until you get your insta-kills or until you get lucky enough to be matched up against bad units like the Gathered Footmen.
 * Don't underestimate the scripted fight in their first territory, some have even said that it's their hardest. I'd advise that you just go all out.
 * Don't be intimidated by the final fight, despite the 50% disadvantage it's easier than you think. Since it's not a castle you start with a relatively small disadvantage in rating, and the disadvantages in Battlefield Effect can be dodged by insta-kill moves, of which you now have 3 (+Dousan who can refill them if needed).
 * Don't underestimate Dousan, some people think that she's only good for Convert Action but that isn't really the case - her stats and troop count are so good that she can be used in the same way that you'd use a warrior unit. Just make sure that you know when to use Youkai Ward and when to use her regular attack.

Uesugi onwards
As I've explained earlier on in this guide, I can't really give you much advice for anything past Uesugi and therefore I won't waste your time here event trying. As for Uesugi itself, you probably already know all that I could tell you. Namely that conquering them before Kenshin is lost is optimal and that while you're at war with the Uesugi House, sending Rance into any battle against Kenshin will cause you to have an automatic victory which can be exploited multiple times to get the Uesugi House's first province almost for free. Oh, and while we're on the subject of Rance, remember that you'll be fighting Takeda soon, so don't forget to make sure that he's got everything that he's going to need in order to do that fight where his unit faces three Takeda general at once.

Also, there is one thing that I should say about the later game. Don't underestimate Tokugawa's final battle, even their lesser uniques like Honda and the ninjas can hit like trucks and if you lose some of your good units to them then you'll be regretting it for the rest of the run. If possible you might want to go down the route of using instant kill units for this battle rather than using the otherwise unstoppable Natori+Kenshin+Uruza+Dousan combo. For this fight your units are just at too much risk for you to try a strategy with is dependent on many different units being alive.
 * On the subject of the Tokugawa House, there's an idea that I've yet to test. In theory, by the time that you have declared war on the Tokugawa House, you will probably have a lot of level one stat books spare, albeit not Speed books. With that in mind, it's fair to say that aside from the +5 SAT that Senhime gives, the only differences between Senhime and Sakakibara Yasumasa are Yasumasa's access to Ally Guard, his 1 point lower NP cost, and his two less points of Spd. Given how hard Tokugawa's castle is to take, this might provide an argument for taking Ieyasu over Senhime in 5SNB. After all, Ally Guard+ is better than All Guard and having access to the other Tokugawa unique units can't hurt. The fact that Yasumasa is recruited from the prison, rather than the map, also makes it quite likely that he'll have a higher inital troop count than Senhime will. Possibly as much as double.
 * On second thought, this might not be a fair comparison. Recruiting Yasumasa does not prevent you from recruiting Senhime. So the comparison should only be between Senhime and Ieyasu, which will tell us to take Senhime every time.

Why you vassal the Tenshi Sect
The first thing that I should explain here isn't so much why you should vassal them, it's why you should keep them alive until you can vassal them. My reasons for this are as follows:
 * 1) Keeping them alive doesn't cost you much to do.
 * 2) *If the Hara House is vassaled then the Tenshi Sect can only attack Kyo, which is easy to defend and isn't even all that big a deal if you somehow lose it.
 * 3) *With only a handful of exceptions, their units are some of the worst in the game and they are therefore easy to defend against.
 * 4) *Only at most one riot can happen per turn and these riots are so easy to repel that I've only ever seen Chousin's unit fail to do it. You can often win the fights just on counterattack damage.
 * 5) *In my experience they don't attack very often.
 * 6) *Once you have the Battle Permits, the Tenshi Sect becomes completely irrelevant.
 * 7) Keeping them alive is beneficial, because it keeps you in a constant state of war. Which allows you to:
 * 8) *Get extra items from winning battles, along with affection and troops for your commanders who participate in these battles.
 * 9) *Avoid other, more problematic wars (e.g. prior to route entry, both Iga and Asai-Asakura's declarations of war are conditioned on whether or not you're currently at war with someone else).
 * 10) *Easily prevent Senhime from leaving you.
 * 11) Keeping them alive is also beneficial for the same reason that we let Hara live for longer than necessary. Conquering or vassaling any house in the game makes all of the other houses stronger.
 * 12) You can turn the riots to your advantage, the riots are so easy to beat that you can send a full army to fight them and use the battle as an opportunity to have your Mikos heal your units.

As for the benefits of vassaling them once you get the option to, as opposed to simply keeping them alive, my explanation depends on what route you're in. For Ran Route players all I have to say is "read on" while for players in routes where the Demon Army appears, my explanation boils down to saying that the benefits listed above will no longer matter and therefore they should be vassaled in order for you to gain whatever benefits that will give you in that route (e.g. Seigan, not having to worry about the Tenshi Sect any more, and extra NP). As for actually giving that explanation, I'll refer to the points I've given above. Point #1 no longer matters because it's not actually a benefit of keeping them alive, point #2 no longer matters because the Demon Army will keep you in a constant state of war, point #3 no longer matters because at this point your army should be so good that you no longer need to care about how tough the Demon Army is, and point #4 no longer matters because money, and therefore healing, shouldn't be a problem at this point.

As for benefits of vassaling them over conquering them before you have the option to vassal them, my explanation also depends on what route you're in. In routes where you can get Seigan then I can give the good explanation of "because Seigan is better than the 5 SAT and the early NP that you'll get from conquering early, and if you're really stuck for NP then you can still conquer Mikan and vassal the Tenshi Sect later on" and in routes where you can't get Seigan than my explanation is again "read on" for Ran Route players while for Isoroku Route players my explanation is the admittedly much weaker "do you really think it's worth the required fans to conquer them and worth losing all the benefits of keeping them alive which I've listed above? Naniwa's also a fairly tough conquer, are you sure that it's worth the effort?".

As for why you vassal them when you're given the option rather than turning it down and conquering them, my explanation is mostly "aside from the difference between the land being conquered and the land being vassaled, they both give the same rewards, so why waste fans conquering them?", however particularly for Ran Route players there's a special benefit to the vassal and it's not just dodging the potential Manmaru Game Over - vassaling them can be used tactically, because enemy houses can't move troops through your vassals, you can let the Tenshi Sect conquer Kyo and therefore (once vassaled) use them as a wall between you and the (Ran Route irrelevant) other half of Japan, making the Ran Route even easier than it normally is.

An Alternative Strategy
In the past I wrote this strategy off as being too risky to even consider on a non-savescumming run, but so many people have reported success with this method that I feel that I would be doing this guide a disservice if I didn't mention it. Remember when I said that the Ashikaga House gets stronger whenever you conquer or vassal the Hara House? It turns out that this is actually true for all houses. That is, if you conquer or vassal any house, all of the other houses will also get stronger. This information has lead to the invention of a strategy that is sometimes refereed to as 'Oil-Miko'. In many ways this is self-explanatory. You start the game, do whatever you'd normally do until you've conquered the Mamushi Oil Field and then instead of vassaling the Hara House and continuing your war with the Ashikaga House, you declare war on the Miko Institute and defeat them. In the same manner, the truly ambitious can either continue on until they've conquered the Uesugi House or they can try to capture some the of Takeda House's Cavalry units. Needless to say, these strategies will put Owari under great risk and Pursuit Battle will be almost a necessity.