joomla users get extended by the vmuser table.
Additionally to that missing data gets completed:
no shoppergroup assigned, when anonymous jUser =>anonymous, else default shoppergroup.
no perm assigned, =>when admin or superadmin jUser =>admin, else shopper perm
So a joomla user can login,.. is automatically a recognised shopper and just need to add his address.
Some people say, vm is not using the joomla account information. They mix joomla account information with the information of a contact detail (com_contact).