What would be the consequences of pissing them off? The fact that they hate me etc.
I was thinking that an unhappy vassal would "rise up in revolution" if you refuse their revocation of the vassalage. You might be thinking "they're so weak why does it matter", however I was thinking of giving them a great general and three military units of the master's technology level per city they own. This would give you some sort of obligation to keep them happy but if you want to abuse them, you should be able to. So I'm not sure. I really want ideas about how to interact with vassals. This is the whole point of the thread.