A Microsoft file hosting and synchronization service.
MS has continued to send me various steps to try and restore two-factor security....resetting, relinking the PC to OneDrive.
Completely uninstalling OneDrive from the PC, then reinstalling and signing into OneDrive and resetting up the Personal Vault.
MS provided a link to reinstall OneDrive - after reinstalling, Personal Vault still defaulted to opening with just a PIN.
I have folders for all of my computer builds - I found an OLDER version of OneDriveSetup.exe (mid-2021) and when I uninstalled and reinstalled with the older version and then went through the sign-in and set up process, TWO-FACTOR authentication was back.
I was initially concerned about all the data I had in OneDrive and in my Personal Vault, however, there were clear options to retain existing data.
I did document all of my findings in my reply to Microsoft.