It might be the cropping bug on some textures that you have in your scene - check the vray render dialog too and it should point you towards an object causing a problem.
hmmm i would start by dropping a gray vray material on all the objects using the global material override or something. just to rule out a material problem or to prove its a material problem. do you use the color correctcorrect at all?
Check all materials (mainly the glass type) in the scene, if you are using any Falloff type maps in the Reflection/Refraction slots of the material (talking about VRayMtls), check on the overide IOR in the options of the Falloff map.
That was a problem with a scene, which created some serious spots, we have examined recently. Checking that tick fixed it.
These’s plants came from an old library where they where created 5 years ago in Maya (before i was here, my boss was working with it) then they where imported into the max library and we keep the original UVW’s that came with the object (using the obj importer in Max).
Putting a brand new UVW modifier on it remove all the spots.
We also notice this problem on some Tree Storm trees.
Finally, this problem is only visible when you use the Adaptive QMC anti-aliasing. Switching to the Adaptive subdivision resolve also the problem but the rendering time was doubled in that case and also, it use to much RAM. This is why I wanted to keep the Adaptive QMC.