The vassal system in C2C, which is mostly just from BtS, is a rigid system that leaves much to be desired.
The system is written as if vassals are enemies that have been 100% subdued, and there's no need to give them anything, or have any further diplomacy with them, and they should not be a factor in further gameplay development other than to be a dead player without yet knowing it.
I don't like anything about it and would probably end up rewriting it entirely if I even attempted to make the smallest change to it, I have a lot of other priorities though so I won't risk trying to make small changes to it as I would end up stuck in a huge project for a long time in all likelihood. ^^