At first I thought my memory (not RAM, human) was playing tricks on me, but now I realized it's a Vegas bug. At least on the 64 bit version Vegas doesn't always save the status of pre-rendered areas. Today I pre-rendered to XDCAM 1080-60i the whole timeline I'm working on, and I'm positive I saved the project after that, and that Vegas didn't crash. Yet, now I just opened the project and the whole timeline doesn't show a single pre-rendered area. Nothing. I have to pre-render all over again.

However, the files that were pre-rendered before are still in the prerender folder. It's just that Vegas is not linking them to the project.