you should have that same texture in the diffuse, then you can see it in sketchup viewport so you can uv-map it with tru-paint, sketchUV or something then turn off texture in the vray diffuse.. it should work.. (that's basically how it works in 3dsmax vray.. if i'm not wrong there is some trick to do the same in vray4SU, like adding the texture to a second diffuse layer and then assignign transparency to it or something like that)