It really depends in the encoder used for rendering. If the codec is not optimized for NVENC or VCE then there isn't much to gain. Also keep in mind that Vegas still relies on OpenCL for FX processing and that just works better with AMD cards.
I have X AVC L and X AVC I files, only Levels and Sharpen are applied on track level and it renders with 40% CPU and 5-12% GPU. I hope they are going to do something with the hardware acceleration. Maybe the problem is that I use Sony AVC/MVC codec, but until there is a color band issue with the MAGIX AVC HD it isn't usable. Even if I render it into MAGIX AVC HD using NVEC isn't very faster... Premiere renders into h.264 way faster using even more vfx and color corrections with the same type of source files.