In one lesson, Kyle Green is rendering a velocity pass from VRay. Just what I'm looking for, but in the link above, Chris Glick is compositing with a motion vector pass from mentalRay. The workflow in interesting, but does not necessarily work with the VRay velocity pass.
Any way we can see how to composite 32 bit normalized vs. full motion unclamped velocity passes from VRay in Nuke?
The same technique should work regardless of the type of vector pass. The only control we would change is Multiply or offset if V-Ray started with a default motion value of 0.5, 0.5 or something. If it did start with a default .5, .5 value, you'd want to set the U and V offset to -0.5 each.
Can you upload a section of the render that is giving you trouble as I don't have access to a V-Ray vector pass at the second.