<a href="http://admin.vbulletin.com" target="_blank">http://admin.vbulletin.com</a> I think, I may be wrong is probably a link to your site from you registration details and for the links database, as I had another hit from there after I changed my details (the URL) in the members area.
The main URL may be forbidden, but there are probably subfolders etc that are not, I've had a few like that myself, try looking through your web server logs (if you have access to them) to find what page exactly, thats what I did.
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