I've seen this too on a laptop where I'm got 8.1 installed on Vista 64. Sometimes when I go to render, almost everything is unavailable. I didn't make a complete list of what wasn't there, but .mp3 was gone, .wmv was gone... It was a bit unnerving at first. I restarted Vegas and they were back... It's happened to me 3 or 4 times (I don't use that machine very often; only when my primary is tied up on something else).
I had a very similar issue with Win7 RC1 64bit / Vegas 9 64bit. Everytime I launched Vegas 9, it would be a gamble to what codecs were loaded and available and which ones were not. Add to that fact that I am using H.264 MP4's with 32bit Quicktime, it was a nightmare.
I am not back on 32bit Win7 with 32bit Vegas 9 and having low memory issues. The fun never ends.