Um, I have to disagree. The entire point of killing someone is to remove them from the game. If they want to sign a surrender treaty were they pay tribute or something thats different. Besides this way would completly ignore the entire Dynasty system. I would like the Dynasty system to actually matter thank you.
Killing the sovereign does not necessarily means killing the player out of the game.
Some random ideas: If your sovereign get his capital captured, then he will probably need to surrender. If the sovereign actully dies. Either the children takes the place and continue to live, or either the kingdom fall in ennemy hands, but the children are exiled and plan a rebellion. Or the children controls the kingdom under their conquerer's command.
In the viktory 2 board game, vassal essentially means:
You cannot attack your boss.
If another player takes your capital, he can free you, or you become his new vassal.
The only problem with that is that the vassal player could get sloppy and not manage very well his kingdom. But there could be ways for the leader to enforce his command to the vassal. For example, the leader could ask for some production/income quota, else he will capture more cities from his vassal. So the vassal would be forced to assure a certain efficiency or get weaker.