OPC bundle includes Virtuemart Cart Reload Plugin which will create a new cookie within the customer browser with much longer expiry date and store the cart content of the user upon each cart change. If the browser window is closed and reopened the Cart Reload plugin will reload the cart contetns (products and addreses) per the latest state of the cart. This plugin can be installed from OPC Extensions tab at OPC backend. This plugin works for anonymous users that are not logged into your site. Within OPC it's possible to disable the VM's core cart reload for logged in users which is enabled on VM3 by default.