If you want to delete the vendor and vendors’ listings then please try to delete the vendor’s user account on the backend or if the vendor deletes the account on the front end then the vendor’s listings are deleted too.
Also if the vendor is deleted in Vendors/Vendors on the backend but the vendor still has a user account and at least one listing which was added before then the user will become a vendor again and all vendors’ listing which was added to/by this user before the vendor account deletion will be assigned to this new vendor account of this user