I do all of my backward compatibility testing for Vegas Pro 8.0 on a Windows 7 64-bit virtual machine so I know it works just fine. I can't say for sure about Windows 8 because I don't use it but it should work as well.
It's running fine in Windows 7 on three of our computers and has been for several years. We also run Vegas 13 on these same computers without any conflict.
I just found that Vegas 7 is not a good idea with Windows 7 (64 bit). After installing it to see how its MPEG 2 rendering compared with later versions, I was unable to open TMPEGEnc Authoring Works (always crashed just before main page appeared).
> "Do you have to run it in some type of "virtual mode" in order for it to run on Windows 7? Sorry, I'm not that familiar."
Sorry for confusing you. I use a Mac so I run Windows 7 in a virtual desktop on OS X. That wasn't the point. Vegas Pro 8.0 has no idea that Windows 7 is running in a virtual machine. It would work just as well on native hardware. Once again, sorry for the confusion.
Yes Vegas 8.0 will work on Windows 8.1, but it will feel nearly as slow as XP if you're using the 32 bit version (small RAM limit before it crashes, extremely slow renders, even on a 6 core). However, if you use Vegas 8.1 64 bit (which came free with Vegas 8.0), you will experience a huge difference (something like 16x faster render times). However, I noticed Vegas 8.1 glitched when rendering into the Cineform format (blacked out part way through the render). I haven't tested this again now that I have the new GoPro Studio (free) installed (which was necessary for newer Vegas versions to even read my Cineform files).
I also haven't tested Vegas 8.1 on full length (one hour project) render, so I don't know if it would black out or render correctly. It renders small clips fine, for sure. It for whatever reason chokes on DNxHD 444 files, playing back at an extremely low framerate, whereas Vegas 8.0c plays the files at full framerate! There's also an annoying bug in the color curves plug-in, where if you save a custom preset, it will only save as a straight line (no effect). Then if you try to delete the preset, you can't. It was for these reasons (and the ~1.3 GB RAM limit on 8.0c) that we switched to using trials of newer 64 bit Vegas versions, and was eventually forced to upgrade to Vegas 13 64 bit. However, Vegas 14 will likely be announced in April at NAB, so if you can wait that would be best. Vegas 13 still has some bugs in it (the Min/Max plug in for example).