a whole bunch of people are having the same issue
follow this
http://sketchupvrayresources.blogspot.ca/2011/07/trick-removing-splotches-in-vray.html

it might also be caustics. i was having the same issue but for me it was glossy reflections that were the problems. it depends on your specific situation. but try it still. i know that vray can be really overwhelming.
cheers