Vietnam's thin defense line and lack of coordination in the offense led to their first loss at the 2020 AFF Suzuki Cup, also their first defeat against Thailand since 2017.
They conceded two goals quickly in the first half to Thailand captain Chanathip Songkrasin.
Chanathip capitalized on a defensive error when defender Nguyen Phong Hong Duy slipped, racing past keeper Tran Nguyen Manh to score the opener in the 14th minute.
He scored his double nine minutes later from the edge of the box after several quick passes.
He was also assigned to take a penalty in the second half but failed to make a hattrick.
The game was fast and tense through out more than 90 minutes, with 39 fouls committed. A total of seven yellow cards were given, including three for Vietnam.
Vietnam created several good chances but they lacked a bit of accuracy.
Nguyen Quang Hai's two beautiful shots were denied by the woodwork. In the first half, his freekick from 25 meters hit the right post. His left-footed effort from the edge of the box in the second half hit the crossbar.
Vietnam were also robbed off some chances as referee Al-Adba Saoud Ali made controversial decisions over fouls by Thailand, including those inside the box.
Defending the title will be a tough challenge for Vietnam now with the defeat. They will play Thailand in the second leg of the semifinal this Sunday.