I have a combined video audio clip from a Nikon pocket camera. How can I separate the audio onto a track of its own so I can edit the audio.

Any "combined video audio clip" you import into Vegas should come in as separate video & audio tracks for more editing as desired.
Are you not seeing this?

I can see them as two separate tracks. However I can't get rid of the very noisy audio.

My first choice would be to delete the audio. When I try to delete the audio the video disappears as well.

My second choice is to edit the audio in Audacity. It edits but I can't seem to get that edited sound to replace the audio portion of the combined audio-video.

Click on the clip to select it and then press the U key to ungroup.

HTH--this one stumped me for awhile too.

I can see them as two separate tracks. However I can't get rid of the very noisy audio.

My first choice would be to delete the audio. When I try to delete the audio the video disappears as well.

The easiest is to click the "Ignore Event Grouping" icon (on the toolbar).
They will now be separate events (the Vegas term for a clip) and can be handled individually.
Another way is to click the audio event and press "U" (Ungroup).

My second choice is to edit the audio in Audacity. It edits but I can't seem to get that edited sound to replace the audio portion of the combined audio-video.

After processing the file in Audacity, save it as a WAV file under a new name.
Import this new file into Vegas, delete the original audio track and put the new one in its place.
Regroup the two by clicking the "Ignore Event Grouping" icon again or by selecting the audio event, shift-clicking the video event and pressing "G" (Group).

Ignore events grouping works. Thank you.

This is another change from previous versions of Vegas. In older versions, the audio could be deleted without the video being deleted even without ungrouping them (or ignoring grouping). The current version can still operate the same way if you wish. Go to Options - Preferences - Editing tab and uncheck the "Cut, copy, and delete grouped events" to change this behavior.

I'd completely forgotten about this one. And yes, I HAVE been u-grouping to delete.

Grazie

I'm ungrouping to delete as well if I only want one deleted. This was changed because so many people were complaining that when they deleted the video, the audio stayed and they didn't want to delete TWICE. Kind of a catch 22 situation but at least we can now each run it the way we prefer.