I just tried the method as you described it again and there's still a problem... here's what I did:
1. open a new empty project
2. drag in a stereo wav file to the timeline
(vegas creates a stereo track from the wav file)
3. right click onto the track, choose "left channel only"
when I now play back the project, I can hear the left-channel-only track in the left and right channel of my speakers. To avoid that I need to
4. set the pan slider to 100% left, but that doubles the output volume of the track, so I have to
5. set the volume slider to 50% as well
If I repeat these steps (except the first) analogous for the right channel I get the desired ability to control the volume of each channel separately. Good.
Where it gets complicated is if I need to apply this procedure to a track in a multitrack matrix project, where the volume levels in vegas etc are already set correctly, and just the balance of a track is messed up a little bit. With this procedure I'd have to readjust many level / effect settings, which isn't impossible but annoying.
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While writing this reply I did another search on the net and I guess I finally found a solution, the free "RS BALANCE PRO" VST plugin which can be found here:
http://www.retrosampling.se/vst.htm (haven't had a chance to try it out yet)