You are in Vietnam, right? I'm going to assume that VAT in Vietnam works in a similar way to VAT in Europe. That might not be true. If in doubt, talk to an accountant.
You can ignore where the users are. The only transaction that matters is the transaction between you and Apple. You sell only to Apple. You handle the sale to Apple in your accounts exactly as any other business-to-business sale.
To which part of Apple do you make your sales? I.e. do you receive your payment from an Apple subsidiary in Vietnam, or do you receive it from a foreign entity? (For example, I am in the UK and I sell to and receive my payments from an Apple subsidiary in Ireland.)
If you receive the payment from an Apple entity that is not in Vietnam, which I think is likely, then it is an international business-to-business transaction. In European VAT accounting this is "reverse charged", which means in practice that there is no VAT. Technically, in the accounts my business both pays and reclaims the same amount of VAT so the balance is zero.