I recently setup a new vCenter 5.1 environment.  One of the first things I noticed was when logging into vCenter it required a full UPN (User Principle Name) or a domain\username format to log in.  In previous versions you could just log in with username & password.  It was a little annoying but I didn't really care as I needed to get everything in vCenter configured.  

I then decided to research how to set it so I didn't have to type in the @domain.com or domain\ format.

I found out this behavior could be changed.  Steps on changing it:

1.  Log in to the vCenter web (can't set this setting from the J# client)

2.  Click on "Administration" from the left menu.

3.  Click on "Sign-On and Discovery"

4.  Click on "Configuration"

5.  Locate your LDAP binding configuration under the "Identity Sources" tab.

6.  Click on your LDAP config then at the top next to the red X there is a button called "add to default domains".  Click It.

7.  You will get a warning about locking out accounts but it is safe to proceed.  Once that is done click the "save" icon at the top and you're good to go.

Logging into vCenter from the web or from the J# client will no longer need the UPN or domain\ format!