vCenter Single Sign by default requires the user to specify the domain when they pass their username during authenticate with vCenter.
For Example : MYDOMAIN\USER1 or USER1@MYDOMAIN.COM
You can eliminate the need to insert the domain in the user name by following the following steps.
NOTE – This has to be done in the WebClient. These options are not available in the desktop client.
- Log in to the vSphere Web Client as email@example.com or as another user with vCenter Single Sign-On administrator privileges.
- Browse to Administration > Single Sign-On > Configuration.
- On the Identity Sources tab, select an identity source and click the Set as Default Domain icon.
In the domain display, the default domain shows (default) in the Domain column.
When you login to vCenter now, you can omit the DOMAIN from your username and connect as just user1.
These are the exact steps as posted in the VMware KB. I added some pretty pictures to assist.
Source : vSphere 5.5 Docs