By "editing" the audio, do you mean applying EQ, Compression, etc, or do you mean actually editing-out bits of it? If you mean editing-out bits of it, then no, you can't do that without re-rendering the video, or else (obviously) the audio is going to be way out of sync with the video.
If you mean just applying EQ, etc., then... is your footage already in Vegas? If so, do your manipulation of the audio (being sure not to change its length), then render only the audio track, to whatever format you want. You can import the rendered audio file back into your Vegas project, replacing your original unedited audio.
There are several additional possible ways of interpreting your quite unspecific original question, so the remarks above apply only if your question meant one of the two scenarios given above.

"then render only the audio track, to whatever format you want. You can import the rendered audio file back into your Vegas project,"

You can render out AC3 as a separate file or multiplexed with video, but you can only import AC3 as part of a multiplexed video.

Thanks for the replies. Here's what I'm trying to do:

Over the years, we've collected several hundred video files on our computers; home videos, weddings, vacations, baby's first birthday, music videos, etc, etc., mostly MPG and MP4. Video quality runs from poor (but sentimental value) all the way to 1080p. Now that we have a nice home theater system, we'd like to transfer the files to the HTPC so they can be played by everyone thru the large screen TV and speakers using Windows Media Center.

The problem is Audio - volume levels are all over the place, some clips have a lot of extraneous noise or chatter and really need to be replaced with narration or music, and some of the older stuff has hiss that is just too audible on large speakers.

So I used Vegas Pro with Sound Forge Pro and used RMS Normalization and Noise Reduction to improve the audio track, and re-rendered the whole file. Now the audio was fine but the video was noticeably lower quality on the large screen. I suspect that may be unavoidable since I'm using compressed files as input, but it may be my lack of understanding of rendering details.

So I was looking for a way to edit or replace the audio track (while keeping it the same length as the original), and then produce an MP4 that contains the original video track but the new audio track. I'm just learning video so I may be missing something simple.

Thanks again for the replies; suggestions welcome.
Dave

MakeMKV should do exactly what you want. It's free.
Preserve the video, mux with alternate or multiple audio, and perfect for home theater playback. Millions of people use it for everything from converting their DVDs to home movies.

MakeMKV looks like a neat little program but as far as I can tell, it only accepts disc type files (_TS, ISO, IFO, DAT, etc) as input. My files are mostly MPG and MP4.

MKV is a wrapper that will accept every common file type known. That is why it was invented.
http://en.wikipedia.org/wiki/Matroska