[tinkerbell]

This is the best you can do as far as I know:-

admincp/vbulletin options/user registration options

Allow Multiple Registrations Per User
Normally, vBulletin will stop users signing up for multiple names by checking for a cookie on the user's machine. If one exists, then the user may not sign up for additional names. Note: This does not stop users from logging out and then registering new accounts.

If you wish to allow your users to sign up for multiple names, then select yes for this option, and they will not be blocked from registering additional usernames.

No

Verify Email address in Registration
If you set this option to 'Yes' new members will not be allowed to post messages until they visit a link that is sent to them in an email when they sign up.

If a user's account is not activated by the user visiting the link, it will remain in the 'Users Awaiting Activation' usergroup.

Yes

Require Unique Email Addresses
The default option is to require unique email addresses for each registered user. This means that no two users can have the same email address. You can disable this requirement by setting this option to 'No'.

Yes

Good luck

Karen