FWIW, I went with SEM paint for the dash. The guy at the paint store said it was "paint" when I was asking around for dye, but seeing it in use, it seems much more like dye. It seems to sort of soak in and evaporate or something, it doesn't sit on the surface. I used the prep stuff they sell too, it supposedly opens up the pores of the vinyl and cleans all the gunk out. It did seem to make it kind of dried out and absorbant.
I'm fairly happy with how it looked after I applied the stuff. I'll post again if anything weird happens or if it wears off or something.