How to Control Transparency on V-Ray material?

[KVMaster]

With a standard Max material you can just turn down a materials Opacity.
But how do you control the Opacity of a V-Ray material?
For example if I have a drinking glass, and I want to put an american flag bitmap on it, how do I make the flag fully opaque (not transparent), but have the glass material it is on be fully transparent with reflections and refractions.

[RErender]

Use a mask in the refraction slot

[mare]

Hi

The same way you control the transparency in standard material, put a bitmap in the refract slot.

Mare