Nothing happens " all of a sudden".  If you get the message "this option is not available" I have to ask you, did you move or re-install your installation of  VERSION 3.71 Holy Smokes!  That's what 12 YEARS old!  Print out the installed modules listing, is VI listed?

Oh, Louie...you're a smart man. (Or a smart woman named Louie.) The whole sad story is that we were all set to make the final move to Sage 100 weekend before last -- everything's installed, tested and ready, just needed the second parallel migration to bring the data up to date -- when we accidentally moved our MAS90 directory into another folder instead of copying it. (Two drive letters had been assigned to different folders on the same server, so the assumption was that the directory would copy when dragged from one "drive" to another. Not so.) It required a decent amount of work to get MAS90 back up and running, but this problem with VI seems to be the final bug. Yes, I printed an installed application listing and VI shows as installed. Again, any guidance is greatly appreciated.

Go into Company maintenance and see if it is activated.  Which version did you "accidently move"  v3.71 or Sage 100?

3.71. VI appears in the Installed Applications list and under Product Registration Maintenance as normal, so it looks installed to me.

I mean activated

Fixed. Files were missing from the VI folder in the main directory. Replaced them from a backup.