Vassals should vote with their liege in Congress

It does add, I guess, more realism. Some people might not be into that, and it could also work as a game option prior to starting a new game.

I think it's not necessarily a bad thing for vassals to have freedom of choice. The problem is that, as it stands, the vassalage system is much more beneficial to the vassals than the suzerain, as vassals suffer a reduction in influence gain, but gain free access to all the suzerain's resources and a guarantee of not be attacked, being able to create cities without fear of conflicts. Meanwhile, the suzerain only earns extra income, which most of the time does not come close to what could be earned with the creation of trade routes, does not generate complaints about the vassals' control of territories, and is still responsible for the problems that the vassal create with other empires.

The vassalage needs to be reformulated, to have more tools to control the vassals, allowing means that in a beneficial relationship for the vassal, they maintain aligned strategies, such as combining attacks and donating territories, using vassals as colonies.