vCenter has independent SSO domain framework which is used to authenticate all VMware products in use in an infrastructure. It include Users/groups and different vCenter services to authenticate with security token based method.
You can create a SSO domain on vcenter, similar to active directory domain where you can create users and groups which will work within that domain only. This VMware SSO model supports integration with other directory services as well like LDAP, Active Directory. So you can add these other services as identity sources in VMware SSO which enable to integrate active directory authentication for vCenter.
For more information, see vSphere Authentication with vCenter Single Sign-On