in the Web UI, can you use the "import from LDAP" feature to see if you can locate the account? This is just to ensure that vCloud Director can locate it. Ensure that you are inside the correct organization you want to login to as well, since LDAP instances are specific to each organization.
By default with Simple Authentication, you have to login with sAMAccoutName (unless you changed the username value). So this is the value you have to search by.
If you can locate it, ensure to import the user and see them in the list (username value should show).
Then log out and ensure to login to the specific Organization URL with the account just imported.