On a completely different track of argument against vassalage in the game, I would have to say that we are playing in a game that takes place during the height of the Industrial Age and not during Feudal Times.
i would recommend giving a game i hate playing "victoria 2" a try, it illustrates very well how politics at this time existed on different layers.
Nations move from "spheres of influence" meaning that the big nations fight for power over smaller ones,
like it still is done today to a varying degree!
I agree that european Feudalism would be a different game than this, i just figured it was the best word i could use to convey message