I agree with the little darth vader...I want the AI to stay alive. Nothing would be better than having Monty as my bi.tch.
This is already feasible, mostly by liberating previously extinct civs. But in any case how would this offer any advantage over existing diplomacy where you have a dependable ally, since the vassal would seem to act the same way?
Vassalage served no real purpose in Civ IV, and it serves no real purpose in the other games I play with a vassalage system (Total War, in which it's made more of a liability still by the fact that vassals will
always revolt at the earliest opportunity once they have a developed military - except in Fall of the Samurai since they share your allegiance, and this is the only TW game that's got a semi-workable vassalage system).
If it can be added to Civ V in a way that works better than most games manage I'd be all for it, but I don't want it 'just because' it's in a previous version of the game and it's not worth building an expansion around.
I think it just isn't very feasible to represent vassalage in games because it's very hard to provide the vassal with an incentive to remain a client state, given the unrealistic but necessary game condition that every faction wants to 'win'. In reality vassalage might often be in the client's interests because they get protection from larger states and not every state has ambitions to be a world power, but in a game where it's usually the player offering protection, you can't really add mechanisms that force the player to protect their vassals, nor can you really have factionss that are content to be minor powers in the player's shadow.
I'd rather have capitulation and vassals which was fun, than city states. City states are the one thing I absolutely despise in Civ 5. I'm too busy with things to do their quests which are annoying anyway, I don't have money to give them and the benefits they provide don't compensate enough for the money they are given, they occupy land mass and if you start conquering them they turn on you. It's just an all around annoying feature, whereas a vassal represented a civilization that was submissive to you, which didn't disappear from the world map and it could always grow big enough to turn on you. It was fun!
I suspect your opinion of CSes is in a minority. I certainly find them an extremely welcome feature. If you're "too busy to do their quests" that's just a matter of your own priorities; for players at large they provide an alternative strategy to achieve victory. Monetary outlay is only likely to be prohibitive if you rely on that exclusively to obtain CS favour (which is very deliberately not intended to be efficient in G&K, which made quests more important and introduced espionage, a cost-free way of obtaining influence). It's easiest to quantify CS favour with militaristic states: say you pay 250 and have a friend for 6 turns or so. In that time, with no other outlay, you'll probably be gifted one unit - even in the game's early stages a Spearman is of equal value to your initial outlay; anything you get on top of that is free money.