Hi
First, check your vCenter object permission.
Go to vSphere Web Client > Home > vCenter > Host and Clusters > highlight the vCenter > Manage Tab > Permissions
Check which user/group has permission/access to the vCenter.
Then check your Identity Source & the default domain for vCenter SSO.
Login with administrator@vsphere.local > Go to vSphere Web Client > Home > Administration > Single-Sign On > Configuration
The default domain configuration will effect on how you use username to login (login behavior).
If the default domain is set for vsphere.local domain, then to login with vsphere.local accounts you do not need to specify the domain e.g. administrator@vsphere.local or vsphere.local\administrator. Just need to input administrator, but you will need to specify the domain name prefix for other domain that is not set for default domain.
See below documentation.
vSphere 5.5 Documentation Center - Set the Default Domain for vCenter Single Sign-On
Each vCenter Single Sign-On identity source is associated with a domain. vCenter Single Sign-On uses the default domain to authenticate a user who logs in without a domain name. Users who belong to a domain that is not the default domain must include the domain name when they log in.
When a user logs in to a vCenter Server system from the vSphere Web Client, the login behavior depends on whether the user is in the default domain.
■ Users who are in the default domain can log in with their user name and password.
■ Users who are in a domain that has been added to vCenter Single Sign-On as an identity source but is not the default domain can log in to vCenter Server but must specify the domain in one of the following ways.
■ Including a domain name prefix, for example, MYDOMAIN\user1
■ Including the domain, for example, user1@mydomain.com
■ Users who are in a domain that is not a vCenter Single Sign-On identity source cannot log in to vCenter Server. If the domain that you add to vCenter Single Sign-On is part of a domain hierarchy, Active Directory determines whether users of other domains in the hierarchy are authenticated or not.