I think this is unlikely to happen (or at least, be sustained very long) in most games because being a slave to another player doesn't sound very fun. Playing as someone else's vassal (and being deprived of any real chance at victory) would be tedious and probably slightly embarrassing. You'd rather just go out of the game, and if in the process you end up screwing over the player who beat you, all the better.

Two powerful players can decide to cease hostilities in order for each to preserve their own ability to ultimately win the game, but if one player's ability to win has been irreparably compromised, then they will probably just fight to the last man in a way that real civilisations pretty much never do.

I think the victor would allow you to even have a small army, not big enough that you can rebel and be a threat, not small enough that you are useless, defenseless (or bored).

And if the victor dies, you would then be free. Not necessarily game ending. And it might match up with actual history too, how many towns have outlived their tributee empires?