Coach Mano Polking has apologised to the Thai fans after the Young Elephants failed to reclaim the SEA Games men's football gold medal.
Forward Nham Manh Dung scored the only goal of the final with a glancing header in front of a capacity 40,000 crowd at Hanoi's My Dinh stadium on Sunday night.
It was the first time that Thailand lost to Vietnam at the SEA Games. In the previous 12 encounters, the Thais won eight and drew four.
"I can only say sorry to the Thai football fans. We came here trying to win the gold medal but could not do it," Polking said after the match.
"But I am also proud of my players because this was a good tournament for them and they played well.
"I hate defeat. But Vietnam were strong and they had the support of 40,000 fans.
"They defended well, played long balls and were patient. They also scored a great goal. Thailand also played well. We created many chances but couldn't score. That's football."
The men's football tournament is for U23 players.
Polking normally coaches the Thai senior side but was appointed by team manager Nualphal Lamsam to take charge of the SEA Games team as a one-off assignment.
Nualphan, who is manager of both the senior and U23 teams, said U23 coach Worrawoot Srimaka was worried that he may not be able to do a good job at the SEA Games following a string of poor results.
Thailand have won the SEA Games title 16 times and Vietnam three.
Meanwhile, thousands of Vietnamese poured into the streets across the country after Sunday's game. Fans waving Vietnamese flags and blasting air horns clogged the roads of Hanoi and beyond in an outpouring of national pride.