How to find out vertica user list?

https://www.vertica.com/docs/latest/HTML/Content/Authoring/SQLReferenceManual/SystemTables/CATALOG/USERS.htm

Also, in vsql you can issue the \du meta-command!

Example:

dbadmin=> \du
         List of users
   User name    | Is Superuser
----------------+--------------
 dbadmin        | t
 mygames         | f
 joe            | f
 ma_dbd         | f
 that_other_one | f
(5 rows)

https://www.vertica.com/docs/latest/HTML/Content/Authoring/ConnectingToVertica/vsql/Meta-Commands/DPATTERNMeta-commands.htm