More than often I have found that when engineers have installed VMware vcenter 5, they have tended to use the same password for the database and the admin@system-domain password. Although this is not good practice it helps to find out the admin@system-domain password as did you know that the database password is kept on the server in plain text!!!
Browse to the path below and open the config.properties file in notepad.
C:\Program Files\VMware\Infrastructure\SSOServer\webapps\lookupservice\WEB-INF\classes\config.properties
Gives you a good place to start anyway.