Bottlenecks including RAM and HDD may be taking over when doing things such as processing effects and video/audio layers. Rendering a file from Vegas is not like simply transcoding a file into another format. There are other things Sony Vegas has to process so things wait for each other naturally - it's not always straightforward. Same goes for Premiere.
Consider placing all your project video files on one HDD and when rendering, write to the other HDD. When reading to one drive and writing to another, the process is sped up because HDD to processor throughput is maximized. I do this a lot. For small projects I use my X-25M SSD as the 'read buffer' since the SSD is much faster for reads. You can resave a project and all it's media files by clicking "Save as" -> and checking "Save media with project" in the dialogue. Be sure to create a new folder. Any related media saves in that folder which when appropriately positioned should be in that HDD you want to read from. Render the project file to the other HDD. With Western Digital 1TB Black HDDs the process should be fast.
Because video files are often stored, moved around, copied, read and written in memory during the render process, ensuring additional bandwidth with a fast memory setup with high speed (low latency also helps on AMD systems) certainly helps with the process.
Cheers; an experienced Vegas user