A method I use is (there may be others?):

1. Lower the gain of the original sound track to zero. (Search for "Event" and look under "Event Envelopes (ASR)" in Vegas help for how-to).

2. Add your music choice to a new audio track.

3. Place your cursor at the point on (note: ON) the original video track where you want the background music to decrease, and the original sound to increase. Press "S" to split the original video (audio should split, also).

3a. Place cursor on the original video where you want the original audio to decrease, and press "S" to split it.

4. Ditto #3 and 3a for the background audio track.

5. You should (the first time you do this) have three events for each track. Now, on the middle events only, adjust the gain for the two audio tracks, increasing the original audio track gain and decreasing the background audio's gain to the desired levels.

6. At the begining and end of each event, you can adjust the amount of fading so the volumes don't just "switch" rapidly. So, you may want to split the original audio a bit early so you can raise it gradually to the point desired. on the background music, you may want to split it a a bit late, and decrease it gradually to the same desired level/point.

Play with it a bit, and you'll catch on quickly. Don't worry: splitting does NOT affect the original contents of any track source: hence the term "non-destructive editing!" I used a simular method to get a portion of my duaghters wedding video to replay several times, snapping not smoothly, but quickly, at the point where her new husband removed her garterbelt and repeat several times to the musinc of "the stripper.!" Cool effect!

The above could be accomplished since each event can be stretched. Try it!