.mtl will not work with this V-Ray. That's a Max format, which I am sure this model was meant for. Now with that said, I have to say that this is the limitation you will hit every time with SketchUp. It just cant handle geometry like this. Try to import the same file in Max and see how it handles. It's just night and day. SketchUp was never meant to be this kind of program. Some rendering engines allow for proxies which puts a low poly placeholder in SketchUp and then renders out the hi-poly version. V-Ray currently does not have this capability but it is in the works for their next release.
M