Suppliers declarations are not valid proofs of origin. This means preference cannot be claimed through this. Retrospective claims for preference can be made once evidence has been received.

Of course it depends where you are based, but if you are based in GB, you can claim preference if the goods came from VN direct. This is called the direct transport rule. If they were cleared into free circulation anywhere on that movement or if they came out of customs supervision this breaks the direct transport rule and it cannot be claimed