I want to make a video with two alternative audio tracks, one with the audio that accompanies the video (native audio) and one option with a panel talking about the video with reduced native audio in the background.

I'm using Vegas 8 and DVD Arch 4.5 on a 70 minute NTSC video on a standard DVD. How can I use the same video and allow the user to select the two audio versions?

Look in the Help Menu in DVD Architect for "Adding Audio Tracks".
It's a section in the "Using DVD Architect Software" chapter under the "Contents" tab.
I'd repeat it here but there's a lot of symbols used that won't show up if I did a copy/paste.