Assuming you had Windows restore on, Windows updates will create restore points. So you might try restoring to a previous point one at a time and see if you can get things going again,
In the future, NEVER do a Windows update or any kind of change(s) to your system without first making a full and confirmed backup of your system. Note the word confirmed! And please include your system specs here to get the best help.
Yeah, well, it was the Windows 8.1 update so it's not really going to be "undone".
I'm more interested in tricks that can get Vegas to behave again.
Ie, switching compatibility modes or something. Vegas 10 is fine. Running as administrator seems to get things further along in the opening process percentage-wise. Same with switching to various other compatibility modes.
[I]... it was the Windows 8.1 update so it's not really going to be "undone".[/I]
That's why one should use a system imaging program (Windows image backup, GHOST 15, Acronis) to back up the system in various states to avoid becoming an operating system and application program debugger [I] some of the time[/I].
I've done upgrades and updates that don't get undone, I just restore the old image and forget it ever happened.
Perhaps others will be more interested in the debugging.
I would look at two things. Your hard drive could be starting to fail, or you accidentally turned on Drive File Indexing. I had the same exact issue and it was fixed when I replaced my OS SATA drive. I also noticed one of media drives was clicking as well and replaced it.
An HDD fails very slowly. Also, take a look at your error codes if any show up, for clues. What does your machine do just as Vegas Pro fails?