Be kindly informed that there will be no problem if you change your arrival airport. Since the visa approval letter works at 05 international airports of Vietnam (Noi Bai in Hanoi, Cat Bi in Hai Phong, Da nang in Da Nang, Cam Ranh in Khanh Hoa and Tan Son Nhat  in Ho Chi Minh City), you still get the visa stamped onto your passport at any of these five airport regardless of which you put in your visa application.