Today a customer registered on my site, but the account was gone when I checked who registered in my shop. Is there a security issue? I haven't seen any option to un-register, at least not in the admin account (which could be the reason).
Registered as a Joomla user? Registered as a shopper? Which?
as a shopper, it must have been as a shopper because the user registration confirmation mail has the format of the Virtuemart mails.
Check index number in database and see if there is any unused number, indicating deletion.
Jörgen