vobs are basically .mpg files, just with non-standard headers and audio mixed in. Any version of tmpgenc should be able to separate the streams for you. Just cancel the wizard it usually tries to throw you into, and choose "MPEG tools" from I think the file menu. Then you can demux the vob and you will end up with separate files for video and audio. It doesn't even do any "converting", really, so you get the exact same mpeg2 stream for the video and whatever format the audio was in.

You would get the best quality by editing the mpeg2 directly, so that it doesn't have to be re-compressed when you're done.
It's hard to find good mpeg2 editors, though. Womble mpeg2-vcr is okay sometimes, but sometimes it's a pain in the ass and can generate non-standard mpegs that no longer play properly in other programs or dvd players. Haven't tried to many others. So you may have to convert the video again to something like a dv avi file, (tmpgenc can do that for you, too), and then you could edit it in pretty much anything you want to. Premiere's kind of a piece of junk, Virtualdub is nice if you're looking for free software.