I’ve got an intricate pattern mapped to a plane as an opacity map. PRT Vol assigned to object and then Frost. Looks great in the viewport, but VRay renders the Frost object as a simple plane. Any idea what’s happening?
Litmus test: Switch to Scanline renderer and check to see if it renders correctly. 
I tested here with a Plane and Cellular Map set to Explicit Mapping Channel 1 and Size 0.1 in the Opacity Map slot of a Standard Material.
Then I converted the Plane to PRT Volume and then the PRT Volume to Frost. Assigned the same material to the Frost and got the right opacity as expected.
Also tested with mental ray and got the expected results. Don’t have VRay here to test though.
Are you sure VRay is rendering the Frost as opposed to the original Plane?
There have been issues with Frost not updating in some renderers, try adding a MeshSelect or any other modifier on the stack above the Frost base object and see if it helps…
Worked with a plane…My file had a box
It seems to work as expected for me (Frost on the left, box on the right):
Is that scene along the right lines? Aside from the “Geometry” meshing mode, I can’t think of anything that would make Frost appear as a simple plane.
frost_opacity.zip (27.3 KB)