To add a Vice Capttain, they first need to joing 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 inlcude 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 Vice-Captain drop down button to assign their VC
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.
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.