After getting the visa approval letter, you can enter Vietnam at any of the 8 international airports in Vietnam that allow this type of visa. These are Noi Bai Airport in Hanoi, Cat Bi airport in Hai Phong, Da Nang airport in Da Nang city, Cam Ranh airport in Khanh Hoa, Lien Khuong airport in Da Lat, Tan Son Nhat airport at Ho Chi Minh City, Can Tho airport in Can Tho city and Phu Quoc airport in Phu Quoc Island.
This does not affect the Vietnam visa application form online. In case you are not sure of the date of immigration, you can fill in the estimated date.
However, you must consider before completing, because you are allowed to enter Vietnam after the date of entry filled in the application, and if you want to enter before that date, you must apply for another approval letter.