Im posting this in case anyone else is having this issue. I recently upgraded from vray for rhino 1.5 to the latest version of v-ray. When I did this one of my files would not open when I had the vray 3 plugin enabled. It turns out there was some conflict between the original vray materials and the new version of vray. Anyway Ill paste the email from vray/ chaos group explaining this below- hopefully this will be of help to anyone with this issue
This was a bug in how material userdata is being read.
In case it happens again, you could apply new materials to all layers / objects and save out the file again. (In your case, I assume at some point you have overwritten the erroneous material)
You could also save out the file as geometry only and then reload it without any issues.
Our developers have managed to implement a solution which will be publicly available as part of the next V-Ray for Rhino update.
If you need any further assistance, please reply to this email.
Chaos Group Support
Quality Assurance Specialist