about vassals, I have some kind of liking for those things:
- they follow your diplomacy
- they give you happiness.
So when I'm in a warmongering game (going for conquest or domination or even "diplomation"), I'll try to vassalize my neighbours asap, being in a near permanent state of war.
The first vassal is costing you a lot :
- city maintenance,
- diplomatic demerit for the other AIs,
- happiness penalty in your vassal's former cities (you didn't kill him!).
But the second one not as much :
- your power and your first vassal's power add up vas your second opponent, making him easier to capitulate
- you get a second happiness bonus for a second vassal, but each city only has a happiness penalty for one former owner

- diplomatic demerit isn't as important when you're getting so powerful
- you gain mutual struggle diplo bonus from the second war with your first vassal
- your "number of cities" maintenance is probably already maxed out
Then the third vassal is really a gift :
- if you traded your military techs to your first 2 vassals and didn't bleed them dry on the money front, the y have upgraded their troops = power to you

. + the techs count for you and for your vassal, making your power absurdly high this way. So the third vassal will face a real force = he will capitulate (depends on personality of course) a lot sooner. Remember that he can not capitulate to your vassals, so you're the only master.
- you now have 3

from vassals. The happiness penalty from motherland certainly isn't a problem anymore. (+ you can build up some culture in those cities, and thus get rid of it totally)
- you certainly now have maxed out the "number of city" maintenance.
- your first vassal certainly is pleased now. The second probably isn't as cold as he was.
- no one will dare attack you anymore, and you can demand all their gold every 10 turns

.
If you attack a peacemonger after that, you may vassalize him by just capturing one city (and waiting 10 turns, so he accepts to talk to you).
I love it when all the world except one guy is my vassal : CONQUEST!