I was unable to open Vegas Pro 18 following a Windows update on Sunday and I decided to reinstall it after I ran out of ideas to try. I uninstalled it using IObit Uninstaller, but when I tried to reinstall VP18, it failed and gave me a message that VP18 was already installed and I could therefore not install it. I manually deleted all files and folders with "Vegas," "Sony," and "Magix" in the name, as well as all registry entries with those words in them. I still got the same error message. I tried CCleaner, Advanced SystemCare, and other software to clean my computer, and they didn't work.

What did work after four days of wrestling with this problem is Microsoft Windows Troubleshooter, which can be used to completely uninstall programs that have been only partially uninstalled. It can be found here:

https://support.microsoft.com/en-us/topic/fix-problems-that-block-programs-from-being-installed-or-removed-cca7d1b6-65a9-3d98-426b-e9f927e1eb4d

I hope this helps someone, and if I have this same problem again in a year and can't remember how I resolved it the first time around, I hope I find this post.