BTW, This 'Force conversion' tool with vassals works only inside religious group, doesn't it? So Christian wont be able to make his own created vassal Christian in e.g., Indian provinces, even converted to Christian.
Just suppose, as Catholic Britany I conquer all Ottomans, then convert ALL their land, then change ALL their culture into Brittons, then I decide to release the Ottos as a roleplaying feature…
and the Ottomans suddenly become Muslim… Hm… MUSLIMS… on the land of 100% Catholic Brittons… What a suicide for a ruler the Ottos should be to see himself as a Muslim vassal there…
This remaining in religious group could be a great feature for 'Release vassal' demand in the Peace menu, when a Winner virtually appoints a ruler in the Peace deal, and the Looser only submits to the Winner's will. But it is nonsense in terms when I (AI) appoint(s) specific vassal ruller to the land.
HOWEVER, if there was an option:
The land has XX provinces of AA religion, and YY provinces of BB religion. Which leader do you want to appoint?
OR (even better)
BB religion dominates over AA religion. Do you want to appoint local majority religion vassal (-1 to -10 unrest for 50 years) or minority religion vassal (+1 to +10 unrest for 50 years).