I understand the frustration that you are most likely going through - a similar thing happened to me sometime ago. In fact, I left the problem project alone for quite a long time. I then found that the non-opening veg file would open after I had first opened Vegas with another veg project of a similar complexity (AVCHD .mts media).

I stumbled upon the problem only a couple of weeks ago. I was scrolling through the Project Media in thumbnail mode and noticed that one of the .mts clips was showing a generic icon rather than a thumbnail picture. Interestingly, it wasn't even a clip that was being used on the timeline, but it was showing under Project Media. After deleting it from Project Media and from the Windows folder, the problem with opening the veg file was solved - and it continues to open successfully. I assume that the .mts clip was corrupt for some reason.

But how to open the veg file to see if something like this might be the cause in your case. Try everything and anything. Reboot your computer and try to open the veg file from within Vegas (i.e not from a pinned selection on the Status Bar). Try first opening another project in Vegas and then, after it has fully loaded, select the problem veg file (but don't first close the already open project). Try rebooting your computer in Safe Mode, and then see if the problem veg file opens in Safe Mode - I did find that this was mostly successful, and I was able to copy/paste the timeline to a second opening of Vegas (you can open VP more than once at the same time in separate windows). I was also able to get an EDL from Vegas in Safe Mode so that I could then "rebuild" the project back in normal Windows (though the corrupt clip eventually caused the rebuild veg file to crash on loading again some time later). Though the latter is now ideal as it is totally boring rebuilding 30 minutes or so from an EDL.

Another suggestion (which I read on this forum a while back) is to delete the .sfl files (including sfl0, sfl1) from your project folders via Windows Explorer, and then open the veg project. If this works, VP will recreate the .sfl files for all the media on the timeline on opening. It might be that an .sfl file is the problem.

As in your case, my project opened successfully for some time before the problem reared its ugly head. Perhaps I opened the problem-child .mts in the Trimmer - thought didn't use it on the timeline - but merely opening it irreversibly caused the problem while that clip remained in the folder.

I really hope that you are able to resolve the problem.

Cheers