hmmmm
lets see. What kind of database do you currently have (with the authentification information)? MSSQL, MySQL?
What you would probably have to do, is set up vBulletin (if you haven't done so yet), and then code some kind of import script which fetches the users from the old database and inputs them in the new one. Don't forget to add a $salt for each user and to encrypt the password with it.
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