Iran had to wait 43 minutes to get off the mark before the floodgates opened against Singapore, who fell apart after Ismail Yunos was sent off in the Group E tie.
On a high after winning their first international soccer title two weeks ago, Vietnam were two up inside 30 minutes while Group D rivals China lost 3-2 to Syria in Aleppo.
Jordan and Thailand drew 0-0 in chilly Amman to share second place in Group E.
The visitors could have collected all three points but Teerasil Dangda and Sutee Suksomkit squandered good scoring chances.
In Tehran, Majid Gholamnejad fired Iran ahead with a stunning strike from outside the area before Karim Bagheri and Gholamreza Rezaei extended their lead soon after halftime.
The hosts made it 4-0 after a long-range effort by Mazyar Zareh and substitute Mohammed Nouri added two late goals.
"The players are getting better and better in terms of tactics and organisation," Iran coach Ali Daei told reporters.
"They are young but they will do good things. I told the players to use this as preparation for more important matches."
Nguyen Minh Phuong and Le Cong Vinh gave Vietnam their early advantage in Hanoi. Nguyen Vu Phong added a third on 69 minutes before Akram Moghrabi pulled one back for Lebanon.
Syria raced into a 3-0 lead inside 39 minutes but had to survive a late fightback by China, who are still recovering from their embarrassing early exit from 2010 World Cup qualifying.
Maher Al Sayed scored twice before Firas al Khatib converted a penalty. Qu Bo reduced the deficit for China on 52 minutes and Jiang Ning added another deep into stoppage time.
"I'm satisfied with the result but the performance was not convincing," Syria coach Fajr Ibrahim said. "Especially because we had a 3-0 lead."
