Worldwide warranty

Don't apologise for your English. It is perfectly understandable.

Your Apple warranty is only valid in the country the phone was bought in. It is possible that other countries may look at it for you but it should be returned to the USA for any repairs.

I am sorry; I don't want to appear unhelpful but we are not allowed to publish our emails or establish personal contacts to resolve problems.

Registered post is available in most countries. It usually requires you to pay an extra amount to cover loss of the item sent. I can't advise you about Vietnam postal matters. You need to enquire of the postal service.

Make sure that whoever you find has ...

A letter of authority from you to make decisions on your behalf.

A copy of the original receipt.

That it is in its original box.

And register the post if you send it by mail.

Then that is as far as I can help you. Good luck.