You can cancel a deal after ten turns. You can get rid of a vassal by making an unreasonable demand that they will refuse, forcing you to go to war. Then you can sue for peace, I think that will break the vassal/master relationship.
You may find this hard however. I have not tried THAT hard, but I have never managed to loose a vassal this way. I have declared war on a strong neighbour of a vassal, and let him take much of the vassals territory. He will usually break the vassal/master relationship so as to make peace (and usually become a vassal of) the neighbour.