Is there any reason that you can't demand something and attack on the same turn even if they give in? It seems there is no downside to accepting the tribute and then killing them anyway. If your relations are good enough, and you are strong enough, you might even be able to get a tech which wouldn't be possible when the war is over and your enemy is no more.
On the other hand, has anyone had the AI do that to them?
On the other hand, has anyone had the AI do that to them?