If you have a vassal they are essentially part of your country. A duke or count is a vassal of a king for instance. If a King declares war on someone his dukes and counts also go to war. If you could have vassals but not be allied with them so they don't go to war it is silly because that means there is territory that is part of your empire that the enemy is unable to attack and sack. If you are France and Orleans is your vassal, why would they not go to war with Burgundy if you do? They are giving you troops and money (vassals increase force limit and income) so they are fair game for the enemy as far as I am concerned. If anything being able to break alliances or not call them into wars is the gamey messed up part.
As others have said, I think the real issue though, is that royal marriages are a requirement. That should be changed to SoI I think.
As others have said, I think the real issue though, is that royal marriages are a requirement. That should be changed to SoI I think.