> AND that would mean being able to click on this proxy file and being able to re-edit in ACID - at the drop of a hat!
Actually you can do this now. If you render your ACID project and imbed the project properties (i.e., use [X] Save project path as reference in rendered media), you can right-click that wave file in Vegas and select Edit Source Project. This will open the project in ACID, you can change and re-render the file and when you switch back to Vegas, the changes will be reflected just like the Edit In Sound Forge option.
Since a proxy file has to generate an audio file anyway, you might as well work with the real one.