Graphics card questions

Hey everyone, so I recently upgraded from the GTX 550ti to the GTX 660ti. I do a ton of editing on sony vegas and use my graphics card to speed up the rendering process. However when I changed over to the 660ti vegas recognizes my card and sees that it is there. But when I go to render it is not using my card at all to speed up the process. Am I doing something wrong? I don't really want to go back to my old card because I am getting more in to PC gaming and the 550ti is a little dated. Thanks for any help in advance