Vassalage already exists in the game with coalitions and even some alliances.
When a game begins, coalitions are created which means several things can happen depending on the leader.
1. The leader can state that they are now a team and will work together to win the match. (My position, incidentally)
2. The leader can declare himself as dictator and NOBODY is to do anything without permission or orders.
3. The leader can institute a system whereby all other coalition players must give him resources as either a tax or whatever because that's how you get into the coalition or stay in the coalition. (In essence, vassalage)
4. I've also seen some coalitions using members as farms for not only resources but even for land.
Basically, this game can be almost anything you like but the onus is on the players.