Answer: For Windows Home Server version 1, if you don't remember it, try each of your email addresses in succession, along with appropriate passwords.
If you still can't discover it, unfortunately there is nothing on your server which will contains the Live ID used. When your server registers for a domain, it gets back a token from Live which it then uses for future communications. You can attempt to contact
Live to see if they will help you, but it will probably be less trouble and faster to just create a new vanity URL.
On Vail, you will find the email address here: HKLM\SOFTWARE\Microsoft\Windows Server\Domain Manager\ActiveConfiguration