If you go to the sony Vegas Website here:http://www.sonycreativesoftware.com/vegaspro/gpuacceleration
Look at the column on the right hand side it says this:
"NVIDIA recommends NVIDIA Quadro for professional applications and recommends use of the latest boards based on the Fermi architecture."
For some reason they think Fermi are the latest cards?
I'm not sure if anything has changed but if you are not able to select it in the drop down then probably not. It has come up a few times now and I dont think it is supported yet...