[Lene Roland]

I haven`t worked with the type of files you`re mentioning, but to get around the settings-issue in Vegas, I´d try deleting the video-tracks, save the project under a new name(to be safe;) and then render the audio.

The finished audio could be synced up to the finished video in a third project when it`s done. I don`t know if it will save you much storage-space, but it should at least save some rendering-time…