Saudi Arabia is the leader of Group B in the qualifications for the World Cup with 16 points after a 1-0 win over last-placed Vietnam.
Only the top two in both of the six-team groups are guaranteed a place in the World Cup in November 2022.
The two third-place finishers go to a playoff with the winner meeting a team from another confederation.
On Tuesday, Saudi Arabia started strongly in Hanoi and after 30 minutes Saleh Al-Shehri’s looping header from inside the area was saved by Bi Tan Truong in the Vietnam goal, but the ball was adjudged to have already crossed the line.
