You can have multiple Team Admins (Vice Captains) of a team.
To add a Team Admin Vice Captain), they first need to join and make themselves known on discord (in new-rider-intros ) , so as to be given Vice Captains role - including updating their Discord Server nickname to include both Club and Team as per the welcome instruction.
New Vice Captain needs to register a VC account on ladder website https://ladder.cycleracing.club/signup
Existing captain goes to their team management page , and should use the Change Team Admins button to assign a Team Admin (Vice Captain)

In the Edit Admins menu, use the drop-down menu and select your Vice Captain.
Note that the list of admins is drag-able, with the top placed name being assigned as Captain.
The list will be of anyone registered as captain/vc on the Ladder website, and who is part of the same ZP club the team was attached to. Otherwise they don't appear.
(Note - we get the Club data from zwiftracing so you need to do an activity that zwiftracing will recognise, for it to pass through to our system)
If they do not appear in this list, please double check that they have completed step #2 above, and that they are registered to the same Zwift Power club that the Ladder team is attached to, as their primary club.