Don't bother. Just right click on the audio track, in Vegas and go to the "Track" tab and choose left only. The other option is to use the balance and drag it down 50% to the right side.
Edit: That's about the 3rd time Spot has beat me by about a minute or two. While writing a response.
Have you checked your Options>Preferences>Audio, and made sure you've got the Echo Asio drivers in there?
Have you checked your Echo control panel to be sure you've got it set to allow mono input?