Vietnam visa approval letter is acceptable at all International airports in Vietnam. It include: Tan Son Nhat (in Ho Chi Minh City), Noi Bai (in Hanoi) and Da Nang (in Da Nang city), Cam Ranh (Nha Trang city), Phu Quoc( Phu Quoc Island). There is no problem when you are arriving at one of these five airports.
