You can't.
identifierForVendor
will only give you the same identifier for apps from the same developer on that device. And it's not guaranteed to be permanent; if you delete all the apps from the vendor, and then reinstall them, it's likely you'll get a different identifier.
As for your iOS6 implementation, that's not going to give you anything permanent, as the point of a UUID is to give you a unique string every time.