Digital intermediates are a better idea - but if you want to use a DI as a proxy case - then I would look for compatibility in one of the proxy scripts already done out there.
In other words you independently create the DI, then sub it into a proxy script to interface as a proxy tool.
I don't think you can speed it up as it basically is a rendering process of each project file. I use Vegasaur for proxy building of my 4K footage because you can select the file type you want. I usually go for XDCAM files because those render very fast on my system and are easy to work with.
Of course, you can speed it up - many ways to skin a cat.
First choose a good DI with accel. speed of render - even the lossless MagicYUV 8bit codec is a very fast codec.
Second take your DI builds outside of the Vegas App - in fact take it out of the entire PC - this is why I use the service of networks for video editing - spread the service load to where it is making actual sense and giving fast results concurrent with all other editing functions.