Okay then, in V-ray, you can set a material double sided (somehow, do not ask me as I do not use V-ray but down in the V-ray subforum on the board index).
This means that while a face can be completely transparent from one side, can be a shadow caster material from the other - i.e. if you create a dummy, shadow caster face for a 2D component, it will result in a "normal" shadow from any angle.
This is actually very useful in SU as well. See an example file by Alan Fraser here:
http://forums.sketchucation.com/viewtopic.php?p=173618#p173618
Although it is not a "classical" 2D face me component, its leaves are actually face me pieces. If you turn hidden geometry on, you will notice what casts the shadow there. You can use that shadow caster component (maybe a bit slanted towards the camera so that you can never see "underneath") with a transparent material on top and a non-transparent, shadow casting material at the bottom in V-ray.
