When logged in to TAC Vista Workstation it is not possible to see any users.
Only a user who has Authority level "System manager" has permission to see users.
If logged in with the system user, make sure that this user has the Authority level "System Manager". If not, try to log in with another System Manager level user. If you do not have one, contact your local Support who will then need a copy of your Vista Database in order to give you a list of all System Manager Users.
There must always be ONE System Manager in a Vista system.