The same "admin" user can log in in every UI. Dashboards are be default only owned by a single person.. which by default would likely be "admin". Other users won't see those dashboards unless they're shared.
If you can't log in with "admin" and can only log in tot he /admin portal, you probably need to change the "admin" password as it has expired.
After you've changed the password you'll be able to log in to the vSphere and Custom UIs.