It's been a while since I've done it but... I remember changing the .vob extension to .mpg and had no problems with it in VEGAS. Hopefully this will be the same for you. -Then again, maybe I'm misunderstanding what you're saying.
If the audio is Dolby ac3 then Vegas can't open it. As B JM says, it only works for MPEG or PCM audio.
If you do a search, you can download a utility called DVD2AVI which will extract Dolby audio and convert it to a wave file. You should be able to use this audio on the Vegas timeline, together with the video from the VOB. Vegas doesn't really like playing MPEG2 files though, so the editing might be a bit jerky.
The other probelm I've had with ac3 audio, particlalry as recorded by that unmentionable Sony camera is there's some odd data blocks in the ac3 stream so tools like BeSweet pass out trying to handle it. In fact most software DVD plaeys always peg out. So you need to run it through ac3Fix first and then DVD2AVI or BeSweet.
But as has been said MANY times pulling video and audio, particularly, ac3 back from DVDs is not a good thing to be doing. Quality is not going to be that good.
The BIG issue that I see is that the general public has not been told this, I suppose maybe the generational loss going from DVD to edit to DVD is no worse than VHS to VHS but they seem to think that because it's in the digital realm there should be no loss at all.
I don't know how many times I've been asked to 'Just transfer it to DVD and we'll edit it later'!