Vendors have bad practices

Vendors suck. Why does anyone let vendors build solutions in our environments with bad practices like using named users instead of service accounts for databases? I have a database that runs under a named user account that was built by a vendor and the named account belongs to a person who's no longer there. So of course we've already terminated the user and they're no longer in AD. If I try to create a new user with the same username of course the SID doesn't match so it can't authenticate to the database to do any owner operations or assign a new db_owner. Of course because we let the vendor manage their stuff in our network they didn't add my team as db_owner I can't do anything in the DB and they've lost the default DB creds which they never provided to us in the first place. If anybody has any ideas how I can get past this problem without having to rebuild the database for an existing production solution I would be eternally grateful. And if you work for a vendor: I'm not mad at you but push back against crap practices like this. If you manage a solution for your clients, in their network or yours, it's your responsibility to maintain appropriate access to data and resources. That means making sure you don't delete the only account which has access to those resources. Same goes for everyone, of course.