Total War: Warhammer - Integrating a vassal via confederation

As Carcassonne of Bretonnia, I manage to enforce a vassal relationship on one of non-playable faction of Bretonnia by taking them by surprise. Which solved several problem at that point in campaign at once.

However I am present with a problem.

I finished all of the tech that enable confederacy, including the tech require for this one single vassal.

So I am wondering is there a way to "integrate" a subject that is a vassal? I am asking because in diplomacy window there is nothing there to suggest a confederacy. At the same time I don't want to break the vassal and have to overcome the diplomacy penalty.

Solution 1:

As implied by this thread, you have to break the vassalage, and then go for a confederacy instead

Searched other threads on this topic and it appears I would need to butter up the vassal, release them from vassalage and then try again afterwards