We are so sorry, you can not.
From 01 Jan, 2015, on the Vietnam Visa approval letter will show the airport you enter. You must enter the airport that you apply and show in that Visa approval letter.
In case you want to change the airport for arrival, you must apply a new Visa approval letter to update the port of entry.
From 1 Mar, 2015, Visa on arrival is available in Phu Quoc airport (Phu Quoc island). Therefore, till now there are 5 international airports in Vietnam are available for obtaining Visa on arrival, including Noi Bai airport (Ha Noi city), Tan Son Nhat airport (Ho Chi Minh City city), Cam Ranh airport (Nha Trang city), Da Nang airport (Da Nang city) and Phu Quoc airport (Phu Quoc island).