Getting vassals of anyone besides a newly former or restored civilization is practicaly impossible.
The AI is simply far more conserned with making you loose than it is with surviving. Even getting a tech or two that wouldn't take more than a couple turns to research is near impossible. They'd rather be destroyed or fall into chaos than agree to your terms. It's so anoying. I can conquer all of western civilization and still not get a vassal, but then be on the verge of implosion, myself. Vassals are prety worthless most of the time anyways. You can''t even demand things from them and I can't for the life of me figure out how to release them without having to goto war with them. If you do goto war with them, they will likely vassalize to your worst enemy and then you have to spend the next 40 turns destroying another civilization. I love the idea of vassalizing other civs, but hate the way ti all works.
In other news, Egypt, Greece, Rome are all prety simple as long as focus on what you're doing and don't get caught up in trying to play for other goals.
babylon is kind of easy as long as everything falls into place, like getting to writing before china, and scooping up near by cities, but if events beyond your control don't work out to your advantage, that's the end of it.
India is kind of the same way. Judaism spawns a few turns before you get it. Too bad, try again. Persia decides they'd like to control your cities and run them into the ground, better hope you can thwart them off while staying focussed on your goals.
Carthage is doable if you can get the right mercenaries. Gotta raze babil and get straight to work on those plantations. Egypt can occassionally be a little too tough to muscle around, though.
England is another one that's not too hard if you plan it right, but because it's more drawn out, there are more things that can go wrong. Don't colonize too early or your economy will be dead, but don't wait too long or all the good spots will be taken. And don't get too cought up in war with Spain or france, britain is a might fortress.
Arabia is actually prety fun, as long as you don't have to goto too far for those holy cities. taking mediolanum isn't nearly as bad as having france and germany breathing down you highly extended neck for a few hundred years. sometimes I don't plan for spains technological advancement and don't get enough troops there, though.
France is okay, not to hard, kind of fun. Again, gotta avoid those warmongering impulses. I had to look up where the cities needed to be founded to get the win, though, it would be nice if all the "control/found/colonize vague region X" could be specified somewhere in the civilopedia, instead of having to parse through victory.py or consts.py.
I've tried Viking only a couple times. It seems like it should be easy enough, but I always come up too short on gold. I guess I need to send more units down into europe to pillage away. I'm just a bit concerned about making THAT many enemies.
I've never played a Japanese game long enough to see how it pans out. I think I get too board with only pety china to deal with, or maybe i realize there's no way I'm going to make the score, either way, I don't think it's all that fun. I really want to play japan into the modern age but never seem to play that far with it.
As Persia I can't seem to get enough land, and those damn huns are so annoying I can't do anything to make samarkand anything besides a money/military sink. you'd think a few spear man could take care of a few horse archers, but they get lucky too often. Also, I find Immortals quite possibly the most worthless UU in the game, especially when babylon is involved. Sometiems the wonders can be a problem too, if china is focusing on them. (I think I may have fixed this, though china still always builds the garden[it doesn't have much else to build when it gets to math])
Spain I've come close, but someone always seems to slip by me, or the aztec defenders get lucky or something else goes wrong. I think it's more fun just to conquer europe, anyway.
Never played germany too long either. Usually end up conquering europe and then getting bored with it. Would be nice to see the UP pay off, though. Maybe I'll make this my next effort.
As Russia I either expand too fast, and loose a city to the damn barbs, or get bogged down in war with the stupid germans that I tire of pretty quickly as they're usually far more powerful than I from the get go.
The chinese UHV isn't all that fun to me. I did it a couple time in previous version and there's jsut too many things that you ahve to do jsut right. I suppose I should do it a couple time to really test my new resource placements, though.
Still haven't played more than a few 20 or so turns as Mali. Never quite been in the mood.
As Inca and Aztec I ahve no idea how you're supposed to keep the europeans out, and watchout if spain is you first contact. ouch.
I haven't been able to figure out how to use the Mongol UP, and am not about to try and conquer far off europe while maintaining a shakey china. Like china itself, I'd rather just fiddle around than go for the UHV. Maybe take out India, and persia if they're still around. Haven't really bothered trying for japan.
Finally America. never done it. either Lady Liberty is already build or close to it, europeans ahve more cities in america than in europe in addition to being a tier or two more advanced than I am, and/or there's no way in hell I'll be able to get all those oil resources. Sometimes Manifest Destiny is a tough enough task. Furthermore, I never seem to get invited to any international conventions.
(sorry if spelling errors bug you, but I've fixed them 3 times now and somehow screwed up the submission of the post, and am not doing it again)