The audio is in the same VOB file; it's just that Vegas does not contain an AC3 decoder;
Pick up a copy of DVD2AVI and use it to rip the audio from the VOB. If it's 5.1 audio you'll also want to pick up a copy of BeSweet (and the GUI for it); that way you can demux the AC3 into 6 seperate WAV files.
here is kinda stupid way...
playback your dvd from dvd player, hook up your DV and capture it back to the computer via IEEE1394.... that will give you DV AVI format
note, not all the dv camcorder has AV IN, look for some sony and canon model that has AV IN/OOUT jack.....