It's great that Pixologic have finally added the function to export VDM's from ZBrush but I can't get a decent result in Mental Ray for 3DS Max (have tested on 2012 and 2013).
World maps seem to work better but tangent maps are more useful as you can deform them and I am currently working on a character animation.
Apart from the main problem with the lack of accuracy, Mental Ray renders out artifacts of black pixels, often forming lines or cuts in the geometry (not related to UV seams).
On the positive side Zbrush VDM's seem to work better than standard height maps but I can't get effects like undercuts to render properly.
I don't have VRAY or anything else to compare it to but I don't think the VDM from ZBrush is the problem as it looks to contain accurate information from what I can see.
Perhaps ZBrush and Mental Ray are not yet properly compatible in the displacement area in general?
If anyone can share any insight from personal experience of the above it would be greatly appreciated.
I am also wondering if NVIDIA have plans to improve the quality of displacement in general using maps generated in apps such as ZBrush.