Thanks for your feedback. I called Mapei and they recommend scarring the surface that the M20 will be applied to, and of course, do NOT recommend scarring VAT.
However, after calling them, I noticed that their Ultraprime WE is designed to go over surfaces including VAT to provide adhesion for their M series overlays. Product sheet:
That is what I was thinking of using for a primer before the M20. Seems that I might be pushing it with this.
I have also done some more looking around, and came across Semco floors. They say that this will go down over VAT, no problem.
Anyone have any experience with this stuff?