The Vietnamese players pressured Lebanon from the get-go, and broke the deadlock after 20 minutes.
After losing the early goal, Lebanon went on the attack, but ended up conceding a further six.
The win maintains Vietnam’s perfect start after beating Bahrain 14-0 and the United Arab Emirates 4-0 to top Group F.
Vietnam will next meet Bangladesh in the last qualifying match on September 23.
The winners of six groups and the best two runners-up will enter the next phase.