The entry gate field looks like a formality. It is not — it is written into your visa and enforced at the border.
Airports
Noi Bai (Hanoi), Tan Son Nhat (Ho Chi Minh City) and Da Nang handle the overwhelming majority of arrivals. Cam Ranh, Phu Quoc, Cat Bi and several regional airports are also on the list.
Land borders
Overland crossings from Cambodia, Laos and China are covered, but only at designated gates. If you plan to cross at Moc Bai, that is what must appear on the application — not a nearby alternative.
Sea ports
Cruise arrivals use designated sea gates. Confirm the port name with the cruise operator before applying; the ship’s itinerary name and the official gate name are not always identical.
What if plans change
You cannot edit an issued e-visa. If your route changes after approval, you will need a new application for the correct gate. This is the strongest argument for booking flights before applying, not the other way round.