"is there a reason you can't just bring your clip into an editing program and just export it as wmv?"
I can't come up with any other conclusion as to why the WMV output in Vegas blanks out after a few minutes and stops rendering audio, other than my 1.66GHz core2duo's not built to handle it. All seems to go well when it renders in Vegas Pro 9 and then during playback the video blanks, and later I lose sound. Shows the amount of noise created by compression (for the remainder of the video) but there is no image. it does the same thing with audio after awhile. No warning - just a supposedly "successful" rendering and bad (or blank) video file.
Maybe in Vegas Pro, everything's supposed to be more professional, so it doesn't give you a warning when parts of the rendering fail - this only happens with WMV. Tried H.264, but QuickTime doesn't shrink to fit into my website's plug-in box. possibly a glitch.
I'll see if they fixed this with the Vegas Pro 9.0e release - http://www.sonycreativesoftware.com/download/updates/vegaspro