I tested Vertex Painting mode with the old Multires, which seemed to work but slowed the viewport at higher levels of subdivision.
The GSoC Multires implementation is where we should be looking, though. When I tried painting in the build at Graphicall, Vertex Painting seemed to only recognize the lowest resolution level (the eight vertices of the startup cube).
I imagine something like partial redraw, and the intelligent automatic type used in the new Multires (and ZBrush), for Vertex Painting. That is, if Multires partial redraw doesn’t do this already. Further, I don’t know where Vertex Painting info is stored in memory, but it may need some attention also – it’d suck to shift multires levels and lose the painting.
With a tighter interfacing of Vertex Painting and Multires, and the new Multires modifier and GLSL viewport, I can’t really think of any reason to continue with ZBrush…