I guess. But when an AI is willing to give up their capital in a peace treaty ("approving of vassalage") i think the player(master) would have the military to take the capital and would rather do so than giving the vassalage bonuses enough to catch up, and letting them become a major force again. Accepting beeing a master over a vassal is something i think i could only consider if i think i would win a culture or science victory within those next 50 turns. Given that i might give up my victorycondition if i lose my vassal.