with youtube downloaders, you'll often get 2 different mp4's now at each resolution for 1080p and below which can be confusing if the downloader has not been updated recently to show the difference. They will normally show you file size, AVC is the larger size which is compatible with vegas, AV1 the smaller size and not compatible.
Not saying you dont' have a technical issue but coincidentally you could have downloaded an AV1 in error to use with Vegas straight after the upgrade
I had lots of strange stuff going on after the upgrade, like my video rendering upside down. Had to do a full reset, and now renders correctly. Hold control-shift down while opening, and clear everything.