You go through the same process to set up your remote access website, enter the same email address for the associated Live ID, and recover your vanity URL and a new certificate for it. Then I believe you reinstall the phone app and set that up again.
Until you've done the first part, your vanity URL may still be pointed to your server, but really that's by accident (your ISP hasn't changed your IP address recently) and the SSL certificate on your server isn't valid for the domain yet.
Note: I don't have a Windows phone any more, so I can't test this. Also, if the Live ID you used for the original setup hasn't been used for anything else, it's possible that Microsoft deleted it (that happens after a period of disuse). In that case, the
original vanity URL is irretrievable.
I'm not on the WHS team, I just post a lot. :)
