I'm currently playing as Russia and "my grandfather" convinced the Byzantine Empress (in her minority) to marry "my father" so I am now the heir to both Byzantine and Russian empires, with 8 Kingdoms to my name. The problem is that I have over a hundred vassals to try and maintain positive relationships with, the vast majority of whom are dukes. The question, then, is it safe to grant a vassal a King-ship in order to reduce the number of vassals to whom I am responsible for managing relationships with - or will I soon be faced with 8 King vassals who all desire to be Independent, and thus fracture my realm right as the Mongols are beginning to invadee?