Kailric
Jack of All Trades
I don't know if my saves would work with your latest version, if you think it can, then I shall try and create a new save for you, it should be fairly easy to premake a situation for a double vassalage.
Does it create the Vassal Lord player, or does a leader join you? I would need a save before the event happens so that it happens on next turn. I should be able to look at the code and tell. I want to get this fixed before I upload next version.
I'll be able to solve this now asap. I can already tell the issue. If you mouse over the two villages one has Vassal owner listed as the Player, the other Vassal Owner is listed as "Vassal Owner", so somehow the wrong Player is being assigned on more than one converts. 
Nightinggale
You could even let primitive tribes start out with all native-like characteristics, and then gradually discover new Civics/Techs to advance to become more full features civs.
