Ok so this might be a dumb question, but does vallejo colors, either VMC or VGC mix well with GW or P3 paints? For some reason I'm thinking that even though they are both acrylics, one is a vinyl base and the other one isn't.
All paints of this type are vinyl paints - they're a development from cell-vinyl paints apparently. And yes, you should have no problems mixing any brands together as the chemistry is essentially the same across the board.
You can mix them with true acrylics too if you needed to, again almost certainly without any ill effects.