Diplomatic Vassal and owned cores.

[Suzaku]

Perhaps instead of a flat -1000 why not get rid of the Malus and instead you return the provinces to the vassal.

While not essential I do believe you should keep cores on the return provinces especially if they're your primary culture or culture group.

 

[t6.28]

What I hate most about that malus is that it totally doesn't matter, if you took the core from them, or if you took it from someone else and they just happen to have a core on it. It often happens to me, that I release a (previously non existent) nation in a peace deal and they don't want to become my vassal, because some of my provinces are also cores of them.