Edit 4: updated to build 527 and allowed Windows to install all deferred updates and I had no difficulty in loading in old projects with surround sound mts video files.
Yesterday I was working on some 13 year old video. Today I had a message that Windows 10 was updating and then I couldn't open my Vegas project. After some trial and error and resetting Vegas I determined that it was only my older video files that were causing the crashes. Any time I attempted to import any of those files, Vegas would crash. This is only affecting Vegas 18 (I am using build 482), not earlier versions of Vegas that I have installed. The video files were created by a Sony Handycam, in mts wrapper, 1080i. I will attempt to upload one of my shorter clips to see if anyone else has similar issue. Right now my Windows 10 is version 20H2.
OS Name: Microsoft Windows 10 Home
Version: 10.0.19042 Build 19042
Edit: problem appears to be focused on latest version of Windows 10 when attempting to import files or load projects that include mts video files with surround sound into either Vegas 17 or 18. No problem with Vegas 16 or earlier.
Edit 2: problem resolved when I uninstalled windows update KB5001330. Another person reported that uninstalling a different windows update number had helped them.
Edit 3: this appears to be the original issue mentioned (in German) https://www.vegascreativesoftware.info/us/forum/vegas-pro-18-build-482-crasht-nach-optionalem-windows-update-kb5000842--128349/
Good news is that Vegas is working on a beta fix, bad news is that whatever it was Microsoft did to break it was rolled into their security fix (mentioned in edit2) and cannot be deferred indefinitely.