Maybe you can get what you want making just a small configuration on the vray frame buffer. I'm not sure but I think it works only for vray 2.0.
When you open the vray frame buffer you'll see a picture of a small gear ( Exposure corrections), you should click on it to enable. After that, you should click on the first button, on bottom left. You'll see a new window and in the top of it you can change de exposition, even if you have the final render. This way you can change the exposition and save the image how many times you want.
If you have doubts about the buttons you can check on this site -
http://www.spot3d.com/vray/help/200R1/vfb_index.html
I hope it helps.