I'm not sure about how automatic you could make it, but i've recently done something similar. Some associates of mine wanted to edit a rather involved project on their own computer, but didn't have the drive space for the 12GB of source clips. I captured all the clips on my computer, then rendered 160x120 15fps Cinepak .avi versions of each clip, resulting in about 400MB total. I sent these clips to them on a CD. When they were done editing, they eMailed the .veg file back to me. I saved this .veg file in the clips' directory, opened it in Vegas, and Vegas promptly announced that it couldn't find the clips. I told it to use the appropriate directory on my drive and it found them all. The final render went quite smoothly.
I suppose what you could do on your computer is create a directory for the full size clips, "source-big", and a directory for the small clips, "source-use", for example. Do all your editing using the clips in "source-use". When you're ready to render, copy the .veg file to the "source-big" directory, rename "source-use" to "source-small" and rename "source-big" to "source-use". If your file names are the same in both directories then Vegas shouldn't even blink an eye; it will use the full size clips the next time you open the project.
the only problem u might run into as i have is if your proxy files are single movie files like avi, or mov, and your source files are an image seq, like tga or bmp or jpg. Vegas does not replace footage from single files to image sequence in the media pool ... it can however, in my expereince, replace any single file with another file very easily.
The way we showed this at NAB (with a 2K wide hi-res image sequence) was to render a DV proxy, add it as a take, and then edit that. Before your final hi-res render, select all, hit "T", and go.
ok ... i stand corrected .... there is a workaround for replacing avi with an image seq. Instead of replacing media in media pool, one can import the image seq, into the media pool and then add as a take to the timeline and then choose that take as a means to replace hi-rez footage over proxy files ... yeah! thanks sonicDennis!