I've used different versions of Vegas in the past, but right now I'm using SV13.
When I place an .avi file on the track, it wont play that files audio. It somehow syncs and combined whatever audio track is already in the project, so I'll ahve overlapping sound, when it should be just the one video file.
I've resorted to converting the .avi file to .mp3 and syncing that with the avi video track, but it gets to be a pain.
It's not that I lay them on top of each on different tracks. But if at first I have a 3 minute mp3 file in, and then I import an hour long avi, for example, the avi audio will be nothing but that three minute mp3 on repeat, with bits of the actual avi audio playing through.