Forcing another nation to release nations generally results in the new nation having the religion of the capital province. In general their government form will be similar to yours (republic/monarchy) but will be the most advanced their tech level allows. There are some exceptions though. In my experience forcing the release of nations is usually not worth the opportunity cost unless the foe is from the same religious group as you are likely to get saddled with a wrong religion, weak, ally that ties up one of your few alliance slots.
Yes now I know, but did not think releasing Byzantium to have a Muslim nation!
Ahh after some time they broke your alliance anyway, very ungrateful!
In the end you can release a nation only to make weak the adversary and then annex the released nation, I've done it with Crimea and worked very well.
I don't think I've the power to annex my fake Byzantium: it's a sacrilege
In the end now I've level 30 and gained the "Nationalism Casus Belli", I've created a strange "Urbino" in Rome and Urbino (forcing Pope to release Rome has no price) and annexed Venice...
A strange thing Modena taken Romagna but it wants that for itself, but it's my vassal!
My other vassal take land for me, how Modena can do this?
(Not a big problem in the end I could diplo-annex Modena with Romagna after!)
I do suppose I've to diplo-annex soon this unloyal vassal in the end
Another question I've 196 relation with an Indian vassal state and the game says I've good chance to diplo-annex that, but if I try it's grayed out it says "not of your religious group", so how I can annex them?