Okay, which leads me to another question: WHYYYYYYY???? Honestly, Vassals are not that good, releasing opm vassals for the sake of it is kind of strange, since they are practically worthless in wars since they don't coordinate really good and get beaten up by every bigger enemy and push your ws down.....
You kidding me? My main armies take out the enemies, and then they do all the sieging for me in enormous stacks. I luv it (i'm lazy though)
Some vassals can even field 15k armies for me (Like Flanders)
EDIT: Also you get half their tax income and half their manpower...which is niceeeh.
EDIT2: In the end it's just to gain enough power to conquer northern golden horde, and my cores there after forming Russia...and then get even stronger.
And to annihilate my enemies, like France.