In vCenter's SSO config, you can declare one external domain to be the default domain. When you do so, you can then login to vCenter using the username without having to specify the domain in which the user principal resides. If your OpenLDAP usernames use the UID attribute and there are special characters actually in the UID attribute, that would be one thing. Or if they're using sAMAccountName. Otherwise, try to configure your SSO default domain.