Vegas has intermediate codecs you can render your video to. Magix Intermediate Codec (a flavor of ProRes, but not officially ProRes (Final Cut won't open it.) is what comes stock with Vegas. You can also render to others such as Cineform, Magic YUV, and many others. Scripts like Happy Otter Scripts (tools4vegas.com) enable you to render to even more flavors. While Vegas won't render to ProRes, it does support opening it.
As far as proxies go, vegas uses XDCAM EX at 720p. Hopefully in the future they will enable us to choose which codecs we want to use for intermediates, but there are scripts (HOS being among them) that let you render proxies in any format and also automate the swapping of proxies with original content.
I'm on a steep learning curve and my terminology is probably dated, date is simply desk top editing :-) the above comments are very helpful, thank you.
Then the answer to your question is, "Any intermediate codec you want."
An intermediate file is usually lossless, not a delivery format, and transfers easily between applications and/or platforms to facilitate shared access.
If that is what you are really asking for, here are a few compared by size, Peak Signal-to-Noise Ratio, and Structural Similarity Index.