Is there a reason why you switched form a very good and well supported card to a less supported card? Anyways, try and rest Vegas to its default settings. Press CTRL+SHIFT and launch Vegas.
Edit: just noticed you are on VP10 so the card model doesn't make any difference at all.
Drivers could matter though. Not sure on Nvidia's card swap procedure, but is it safe to assume you uninstalled the driver BEFORE you install the new card? Might need to reinstall any GPU stuff Vegas needs like DX.