Because Rhino 5.0 is 64-bit but older, avoid massive 8K textures. Stick to (2048x2048) for optimal performance. Package your materials as a custom toolbar using V-Ray Material Library packing tool.
Sometimes, materials render completely black in Rhino 5.0 x64. This is usually due to or normals . In Rhino, type Flip to reverse surface direction. V-Ray only renders the front side of a normal unless "Double Sided" is checked. Rhinoceros 5.0 x64 VRAY MATERIALS