you can bake the mesh in Rhino and it will retain vertex color in the viewports (although you might want it to be more subdivided, maybe through Weavebird) but as far as I know there’s no way in Rhino -at the moment- to combine mesh vertex colors with other materials
this shows you how to enable Cycles mesh vertex color material:
maybe rendering it as purely metallic then rendering purely vertex colors and then somehow combining them in photoshop or similar?