You will get your visa stamped at one of 3 international airports(Hanoi with Noi Bai Airport, Da Nang with Da Nang International airport, Ho Chi Minh City or Saigon with Tan Son Nhat airport).

Once you reach Vietnam airport, just go to landing visa counter. Please check out the map:

Hanoi Airport Map – Hanoi airport visa-on-arrival counter.
Ho Chi Minh city airport map – Ho Chi Minh city airport visa-on-arrival counter