Always check the culture of a AI that wants to peacevasall to you... and if you're expecting a long game, better check if it has Sistine Chapel. Not only might that AI win by culture (which is actually the biggest treat from peacevassaling AIs), they'll also might culture pressure your cities. Although you won't lose tiles, your cities might go into revolt. Happened to me once, and it annoyed the crab (yes, lost my crabs with that every few turns, which was serious -health) out of me.
Unless the vasall wins culture, you're not in danger that way, but it's indeed annoying.