Have you tried ‘packing’ your v-ray projects before uploading them to the cloud? That way the material files are copied into one directory no matter their original source. They still path to their original locations - which for V-Ray materials is fine as it should be the same directory for everyone. For custom Materials, you can re-path from that directory. I know it’s an extra step unfortunately there’s no other way for V-Ray to know where to look for them without pointing to their location manually.