If you open a COPY of the audio track in Sound Forge, when you save it there, it will automatically add it to your Vegas project, as a new take. That way, if you want to revert back to the original file, just right click the file and select Take 1.

Not sure how it handles it if you open the file directly in Sound Forge. I would guess it forces you to update the file in the project, as you just changed the original source file.