I was thinking about fixing vassals, especially for slannesh and Nakai, and I think the only way (and easiest) might be to take away their ability declare war or be declared upon.
That seems the most historically accurate way of doing it and would make it beneficial to have them. They follow where their liege lord goes.
So while they are vassals they are always at war with every faction you are and if you make peace they are removed from the war as well. You wouldn't even be able to uncheck the invite allies for them.
Effectively binding them to you as long as they are willing to pay you for the protection. Could even as an option where you would pledge to another faction in exchange for their protection.
What do you think, agree or do you have another idea?
I like all the races. Equally. Wood elves are just the first among equals.