I don't like having vassal's most of the time. I'd rather just kill off the Civ and take their land and run it myself. There are a few times when I'll accept a vassal though - They do help out in wars and certain Civ's make good tech partners. Still, I think this would be a more powerful option if you could cancel the vassal agreement.
Correct me if I'm wrong, but a vassal Civ's score only counts 50% toward your score I thought... not positive on that, but I thought it was something like that.