Thailand will have to win the second leg of the Asean Mitsubishi Electric Cup 2024 final by two goals if they are to claim a third successive title and eighth overall on Sunday.
The defending champions lost 2-1 in the first leg match in Vietnam on Thursday night.
A second-half brace from Brazil-born Nguyen Xuan Son handed Vietnam a slight advantage in front of a full house crowd at Viet Tri Stadium.
Chalermsak Aukkee pulled a goal back for Thailand as the War Elephants set up an intriguing second leg at the Rajamangala National Stadium on Sunday.
Thailand coach Masatada Ishii admitted his players made some sloppy errors during an absorbing encounter in Phu Tho but also acknowledged that his team had a hard time coping with Xuan Son.
"Everyone knows that Nguyen Xuan Son is a very good player. Our team made several sloppy errors which led to him scoring the goals," said the Japanese tactician.
"Having Nguyen Xuan Son in the team certainly makes a huge difference for Vietnam," he added.
"This defeat is different from the one we had against the Philippines [in the semi-finals] because tonight we went two goals behind and pulled one back but against the Philippines we equalised before they scored again."
Ishii, however, refused to comment when asked about the match referee and simply said, "I respected the referee's decisions."
It was a match that saw Thailand dominate possession but Vietnam showed that they can be clinical and don't need much of the ball to get the result they need.
After a scoreless first half, when both goalkeepers were barely tested, the breakthrough for Vietnam came in the 59th minute when a deep cross from Nguyen Quang Hai found Vu Van Thanh, whose well-placed header set up an onrushing Xuan Son for a close-range finish.
Xuan Son doubled Vietnam's lead on 73 minutes when he dispossessed centre-back Chalermsak on the halfway line and beat Pansa Hemviboon before drilling the ball past Thailand goalkeeper Patiwat Khammai into the bottom corner.
A Suphanat Mueanta free-kick should have brought Thailand back into the game, but it bounced off the underside of the bar and out.
Chalermsak atoned his earlier mistake with an 82nd-minute header to give the War Elephants a lifeline ahead of the second leg in Bangkok on Sunday.
"Today, we did not get a good result but we still have one match at home, but we will try to do our best to win the trophy," said midfielder Ekanit Panya, who played his first game of this championship.
"I'm very happy to be back playing for the national team again," he added.
Thailand now have to do what they did against the Philippines in the semi-finals and overturn a first-leg defeat if they are to win their eighth Asean Championship.
The all-important second leg, which will kick off at 8pm, will be broadcast live on Thai Rath TV (32), TrueSport 2, AIS Play and BG Sports Youtube Channel.