You configured vCenter to use Entra ID for authentication and would like to go back to using Active Directory.
vCenter 8.0
vCenter Server supports one configured external identity provider and switching authentication to Entra ID requires changing the provider. As a result, the previous provider is no longer available.
To switch back to using Active Directory, select Other Providers > Embedded then you can add the previous Active Directory configuration.
Configure vCenter Server Identity Provider Federation for Microsoft Entra ID
https://techdocs.broadcom.com/us/en/vmware-cis/vsphere/vsphere/8-0/vsphere-authentication-8-0/vsphere-authentication-with-vcenter-single-sign-on-authentication/configuring-vcenter-server-identity-provider-federation-authentication/configure-vcenter-server-identity-provider-federation-for-azure-ad-authentication.html
vCenter Server supports only one configured external identity provider (one source), and the vsphere.local identity source (local source). You cannot use multiple external identity providers. vCenter Server Identity Provider Federation uses OpenID Connect (OIDC) for user login to vCenter Server.