Is there a way in Vertica for a user to query v_catalog?

Assign SYSMONITOR role to the user? https://www.vertica.com/docs/9.2.x/HTML/Content/Authoring/AdministratorsGuide/DBUsersAndPrivileges/Roles/SYSMONITORROLE.htm
However, this also gives access to read all system tables as well. You can verify by the following query:

select table_name, is_monitorable from system_tables where table_name='current_session';
table_name       | is_monitorable
--------------------------------
current_session  | t

The t in the is_monitorable column indicates the current_session system table is accessible by the SYSMONITOR.