I've noticed the same graininess with bumped area's. Bumps and discplacement in Vue are best done with normal maps. Just check the displacement block in bump settings and load your normal map in the function editor. I use Pixplant 2 to make normal maps. make sure u set to object paramatric, for accurate results. Here are some good links for tutorials.
Geekatplayβ’ Studio, Resources for 3D Artists. Tutorials.
Tutorials for the 3D Artists. Largest collection of original content from Geekatplay Studio by Vladimir Chopine.
(www.geekatplay.com)
http://www.renderosity.com ( search tutorial section On Vue )
Greetz