Yes, admin can control vendor's approval after the creation of any new vendor. Only after admin's approval vendor will be able to open his dashboard. We provide a configuration in admin area from where you can enable/disable this setting related to approval of vendor.
In admin panel got to System -> Configuration -> Vendor Configuration Tab -> Vendor Account Option (Section) and change the option “Require Admin Approval” to “No/Yes”. It's not necessary to approve vendor after the creation of each vendor it depends on admin.