Extract the audio with virtualdub or virtualdubmod and convert it with eac3to to something else. My guess is that it's 24bit PCM so avisource can't quite cope with it. You could convert it to 16bit PCM for editing, but for the final encode (since the downsampling is a bit of a shame), you mux the original audio back, or perhaps you encode it with another codec that can handle 24bit just fine (flac for example ─ mp3 isn't a choice, aac should be) and mux that one. If you have to cut the audio in vegas, then cutting and muxing the audio would get more complex: you could technically try and match the cuts on the original source with trim in avisynth, and then split the audio with split_aud.pl, but it might really become too complex for what it's worth, so in the case that vegas really does not support 24bit I guess the best choice is just go with 16bit (which does sound odd to me though, I'm pretty sure that Premiere can handle 24bit audio, and it would seem weird that Vegas didn't...).
This is all assuming you actually need to use that audio. If you don't need the audio, then in virtualdub, just pick audio > no audio, and direct stream copy the video, and you'll have an avi without the audio in the way, so avisource won't give you troubles.