The Vietnam Football Federation Referee Committee determined that Lan's mistake affected the outcome of the 12th round match on August 14, which Hanoi ended up winning 2-1.
In the 33rd minute Hanoi defender Do Duy Manh stepped on the foot of HAGL striker Bruno Henrique in the penalty area.
Despite having a clear view, Lan did not spot the foul and let the game continue.
HAGL should have got a penalty and Manh should have been sent off because he had already received a yellow card.
After the match Lan admitted his mistake to the head of the committee, Duong Van Hien, explaining that the action had been too fast and both players had been wearing white boots.
Three minutes later Lan was again in the thick of things when he denied Hanoi a clear penalty after their striker Tonci Mujan was fouled by goalkeeper Huynh Tuan Linh inside the box.
Lan is the only FIFA referee in Vietnam to reach the Elite level, the highest in the hierarchy.
He is considered the best referee in Vietnam.
With their win against HAGL, Hanoi remained on top of the V. League with 26 points, six clear of second-placed SLNA.