Vray2SidedMtl and transparency strange cut-off

[RE:FORM_STUDIOS]

Hi there, Hope someone can give me some quick help. I have a 2 sided material with a 50% transparent and glossy material applied to a plane but I am getting a strange cut-off between nice 2sided effect, and completely opaque/reflective.
I've determined that the reflection settings are not the problem, it's something to do with the transparency of the material,.
My object is a single flat plane, no thickness.

Any ideas how to fix the problem?

[RE:FORM_STUDIOS]

Ok, as per usual, posting the question on here miraculously leads me to find a solution by myself. Turns out the IOR of the material has to be 1.0 otherwise you get the falloff... strange as I'd always assumed a IOR of 1 was BAD.