Here's the offending result:
So the red line indicates the boundaries of the geometry and the green line is the boundaries of the opacity map's opaque-ness.
You can see it's semi transparent instead of transparent. Is it something to do with picking up shadows from somewhere? I thought it might be a vray object-properties issue but can't really find anything significant... maybe telling vray to not cast shadows from opacity map alpha... *shrug*
Here's the opacity map:
