When enabling CUDA Acceleration on Sony Vegas, it says that CUDA is available and it also recognises my GPU (Gainward Phantom 2GB GTX 760). However after clicking render, it crashes almost immediately but I've watched videos where GTX 660s and older cards work fine. Some people said its to do with drivers. I've overclocked my i5-4670k to 4.0GHz but I might have to further overclock it because the rendering is still a bit too slow for me when using the CPU only.
Have you tried updating the GPU drivers or rolling them back to an earlier version to see if that helps with the crashes? Wisecraker asked a couple of good questions about the source file and what output (render) options you are using.