I would like to say I was at war with one civ and that civ was at war with 1 another. They became the vassal of the other one while doing a double front war.

The diplomacy isn't that advanced enough on 3 side war as the 1st winner take all.

Following that statement it should have some improvement in war contribution.

The newly vassal (enemy side) keep trepassing and attack my vassal unit forcing them to auto retreat and decrease my war support while i can attack their vassal and go in their land even with closed border. I just can't go in "crownland of their overlord" I also get the penalty attacked me while they clearly trepass in my core land with closed border.

Reproductible 5/5 : get player empire, 1 vassal under the player
second empire has 1 vassal too (the second empire get 2 war at the same time but will get vassalised by the third empire)
third empire at war with the second

So i would like to say the diplomacy isn't ready yet.