I’m having the same problem, but haven’t ever used anything but Vegas 5, so it can’t be the reason previously mentioned here (that once something has been edited with a later version, it can’t be opened with an earlier version).
In fact, I open Vegas, put single picture (jpg) in, and save it, then try to re-open it, and get that error message saying “An Exception has occured.”
What in the heck is going on? This has now cost me 2 full days trying to fix. and losing all my work.