It will take time to find the scene - it is in the archives somewhere. We have not had any other projects recently that forced us to use the clipper.
As far as remember - Vray version was 3.0.X - not sure the final number, we keep updating to very last version.
Really quick test - different scene units- mm, cm, inches and meters, same objects - 2 boxes of same size, three scenarios - faces of two boxes and clipper - coplanar, clipper and face of one box -coplanar, and third scenario - clipper and faces not coplanar. ( no Xform applied) !
Of what we see from this fairly simple scene - looks like we have just had the luck to be in scenario with no artifacts! 
Looks like 3ds max issue to us!?
Maybe sort of a width added to clipper might be a solution?!