[LupusFatalis]

5 Vassals allows for more vassals than we have normals. It also allows for ~1/4 of the players to win simultaneously? It gives no incentive for an overlord to keep all of his vassals alive, he has a spare slot even. Making it a permanent choice locks in those incentives and removes the ambiguity that they'll have with normal allies. And as I stated, if they have to fulfill win conditions together, being able to opt-out allows for a normal to fulfill normal win conditions as a vassal, opt out and win.

The swallowing a vassal to become a vassal idea is interesting, but not sure I like it--need to consider it more.