The lower image does look cleaner, but the grain is still there. Yeah less bump means less render time - always. Also since this is HDRI you are going to get some artifacts from that too unless you set those samples higher as well. I don't have Vue5, but I am patiently waiting for them to upgrade Vue pro with these features. I'd take a crack at your scenefile, but I don't have Vue5. Usually more samples fixes the grain - doesn't seem so this time. What if you just light it with the sun and no HDRI - do you still get all that grain for the same settings?