Rhino Material Failing to Render in VRAY - objects disappear


I am trying to render a model that contains a mix of rhino and Vray materials, which hasn’t been an issue so far. However I have some simple polygon trees that will not render in VRAY - the whole group just disappears.

It will however, render just fine with rhinoRender.

Any thoughts?

It may be related to meshes, or…
See this discussion: http://forums.chaosgroup.com/showthread.php?80990-Block-Instance-Objects-Disappearing