We have a couple files that were checked out and not checked in by people that are no longer with the company and their login's no longer exist. What is the procedure to check these parts in or can it be done?
For absolute minimum loss of data, Admin would be able to login to the vault on the same physical computer and check in the files. (BE CAREFUL not to "get" file versions from the vault in any way.)
If that is not worth the trouble (or the computer no longer exists and therefore is impossible), *AND* the files have been actually checked in previously, then Admin (and in your environment, perhaps anybody else with appropriate rights), can "Undo Checkout".
If the files were never actually checked in (a.k.a. "private state" ) then *MAYBE* you could pull a copy off your archive server, otherwise the files are gone forever.
Retrieving data ...