Vietnam took a huge step towards being crowned Asean champions for the first time as they scored twice late in the first half to stun Thailand in the first leg of the AFF Suzuki Cup final in Bangkok.
The Thais dominated the first half at Rajamangala Stadium but were caught out by a header from Nguyen Vu Phong in the 40th minute and a tap-in by Le Cong Vinh two minutes later.
Substitute Ronnachai Rangsiyo pulled a goal back for the three-time champions immediately after coming on in the 75th minute but the home side were denied an equaliser when Teerasil Dangda was incorrectly ruled offside a minute later.
The Thais were looking to make the most of home ground advantage and went close after nine minutes when Datsakorn Thonglao centred to an unmarked Natthapong Samana, who sent his header straight at Duong Hong Son.
Four minutes later, a cross from the right by Datsakorn was deflected into the air by Le Phuoc Tu to an unattended Sutee Suksomkit, whose header from 10 metres out was brilliantly turned aside by Hong Son.
The Vietnamese were on the back foot for most of the half but grabbed the opening goal against the run of play.
Le Tan Tai took the ball down the left flank and crossed to the near post where Vu Phong ghosted in front of Natthapong Samana to glance a header across goalkeeper Kosin Hathairattanakool into the far corner of the net.
The Thais pushed forward in numbers but were caught out again two minutes later when Le Tan Tai won a 50-50 challenge near the halfway line against Nataporn Phanrit, allowing Nguyen Viet Thang to break clear.
He was quickly closed down by Natthapong and an advancing Kosin but coolly slid the ball across the Thai box to an unattended Cong Vinh who had the simple task of tucking the ball into the unguarded net.
The Thais had a big shout for a penalty in the 54th minute when Datsakorn appeared to be charged over by Quanc Cuong in the box and 10 minutes later, Surat Sukha fired in a shot from 20 metres that produced a great save from Hong Son.
Ronnachai came on for Teerathep with 16 minutes remaining and made an immediate impact, meeting a cross by Natthapong with a header that forced a good save out of Hong Son but came back off the post and hit the goalkeeper's leg before going in.
The Thais though they had the equaliser just a minute later when Teerasil turned in another cross by Natthapong but the striker was wrongly adjudged by the assistant referee to be in an offside position.
Hong Song made another fine save to keep out a 25-metre free-kick by substitute Arthit Sunthornphit with eight minutes remaining as the Vietnamese held on for victory, leaving the Thais with plenty of work to do in the return match in Hanoi on Sunday.