Vassal in dungeon is a real PITA

[DJIM]

If he usurps something from you, you get a claim on it so you can take it back immediately at no penalty. After a vassal revolts, start by revoking his duchies. This will unlock plots to revoke his counties. If you can pull them off, you can revoke all his titles eventually.

You get plots to revoke counties that are outside their de jure Dukedom