Until recently, Vegas did not have a “smart render” capability.
Everything always had to be re-rendered (even if it was just straight cuts) which, as you know, takes time and affects the quality 🙁
That’s why folks were using programs like MPEG video wizard DVD of Womble or VideoReDo – to get around this limitation.
Vegas Pro 8 added “smart rendering” support for HDV 1080i and XDCAM.
What wasn’t documented is that, under the right circumstances, this feature also works for standard def MPEG files.
As long as your MPEG files were all encoded with the same bitrate, you should be able to smart render and not have Vegas recompress anything.
Note that I said “should be able to” because, if the stars aren’t perfectly that day, it may not work.
I’ve done it and it’s worked for me but other users haven’t had much luck.
Most of my MPEG encoding is done according to templates that I’ve saved.
That way, I know what the encoding parameters were and can duplicate them easily.