Question is it better to render video with cpu or gpu in vegas pro

The problem with using GPU based encoding is what GPUs can do are limited. For the same quality, you're likely to have an inflated file size compared to using a CPU based encoder. I recently did a test with this on a 1080p Blu-Ray rip in Handbrake. The x264 output was half the size of the NVENC one.

Speed, quality, and file size, choose two.