When a user logs in with just a user name, vCenter Single Sign-On checks in the default identity source whether that user can authenticate. When a user logs in and includes the domain name in the login screen, vCenter Single Sign-On checks the specified domain if that domain has been added as an identity source. You can add identity sources, remove identity sources, and change the default.

You configure vCenter Single Sign-On from the vSphere Client. To configure vCenter Single Sign-On, you must have vCenter Single Sign-On administrator privileges. Having vCenter Single Sign-On administrator privileges is different from having the Administrator role on vCenter Server or ESXi. In a new installation, only the vCenter Single Sign-On administrator (administrator@vsphere.local by default) can authenticate to vCenter Single Sign-On.