and there seems to be a vassal cap of four upon conversion (to test this, I killed Karloman before converting, and when I did I saw I only had four vassals. I did a second convert without killing him, and saw both of us each had four vassals, with three of his being ones I had in the previous game). So, to fix this, I would suggest having centralization affect how many vassals you get: 8 being at Decentralized and 0 being at centralized and moving at spaces of two between each law. I also say having Autonomous Crown Authority should give you marches, and Low give you regular vassals.