I’m not sure I understand what “restored Postfix from backup” means. You overwrote your current /etc/postfix/virtual with an old one? But, why?
I think that turning off mail for a user and then turning it back on will recreate their entry (or entries) in the virtual map, but I’m not sure if that’s safe (i.e. won’t delete mailboxes or whatever). You should test first, but if it does do the right thing, you could use the virtualmin command with the modify-user subcommand to do it in a script so you don’t have to click through the GUI for all the users. modify-user – Virtualmin