I had something like this years ago, and if I recall the problem was in the Windows Sound Mapper. I think that I had two inputs selected somehow. VLC uses its own internal drivers and ignores all Windows drivers.
If you have installed a codec pack, then wipe your disk and start over. OK, maybe not that drastic but codec packs are designed for games – fast playback. The codec pack usually also overwrites the good codecs on your system with really inferior ones. After uninstalling the codec pack, reinstall Vegas to reinstall the codecs. I know this fixes video codecs, but I’m not sure of audio codecs.
Steve Mann
MannMade Digital Video
http://www.mmdv.com