They don't obey nor listen to you, just wipe them until they have the last province [make sure that province is their home province] and make them vassal! best way to create economic even in the realm divided situation, its useful option if they not trade with you or distrust you. They even used for defense line from the direction you don't attention too.

BTW, don't let's your vassal have more than 2 province, its have high chance they will start to rebellion against your.