so basically, you are saying the tracks in Vegas don't mix, they just keep amplifying on top of each other?
I'm not sure what you mean by 'mix' - it sounds like what you want is that when you drop a new track into your matrix, you want the program to drop the level of each track so that their combined output never goes over 0dB - right?
But I'm not sure how this could work in practice, unless you always wanted an arbitrary 50:50 mix of your two sources - not something that would sound good in all, or many, cases.
What I would do is:
-drop your tracks into Vegas (and do all your time-stretching etc)
-play the whole thing through with the master control backed off to the point where you aren't clipping/distorting
-use the volume envelope on one or both of your tracks to adjust the relative mix so that the best sound is achieved throughout
-re-adjust the master slider so that you're peaking around 0dB
-export your matrix as a stereo wav
It can be a long, tedious process but the benefit of mixing your tracks like this is that if, for example, you were mixing soundboard and audience sources, you could, say, dip the audience tape during quieter songs when the crowd might be getting intrusive and boost it at the end of songs to get a bit more audience appreciation in there.
Of course, I may have totally misunderstood what you were asking, in which case please move along, nothing to see here...