Vietnam opened the scoring in the sixth and 13th minutes of the match thanks to Pham Van Thanh and Ho Tuan Tai.
However, before the first half closed, Thailand levelled the score at 2-2.
After the break, Lam Ti Phong and Pham Xuan Manh opened the gap for Vietnam to 4-2 in the 49th and 56th minute.
With this win, U21 Vietnam temporarily took the lead in Group A with three points, followed by U21 Singapore and U21 Thailand.
In other Group B match on November 22, U19 the Republic of Korea (RoK) crushed U21 Myanmar 2-0 to top the group and secured a ticket to compete in the semi-finals.
|Opening win: U21 Viet Nam celebrate beating Thailand at the Thanh Nien U21 International Football Tournament in Ho Chi Minh City on November 22. — Photo thethaovietnam.vn
Coach Eric Williams of Myanmar called together the players from the U19 team that defeated U19 Vietnam in the final of Hassanal Bolkiah last year, as well as other players who competed in the 2015 World Cup U20.
Myanmar's experience was better than the youngsters from RoK, so they created many opportunities to score in the first half.
In the 40th minute, Than Paing coolly launched his header towards RoK's goalkeeper at a distance of 5m, but the RoK goalie saved the goal.
However, RoK players did not miss chances in the final minutes of the first half, as Kang Ji-hun overcame Myanmar's goalie to open their score in the 45th minute.
Myanmar played attack continuously to level the score in the second half, but they had to surrender to the RoK's defence line.
RoK proved very dangerous in counter-attack situations. Kim Jeong-hwan doubled the score to 2-0 for his team in the 70th minute.
In the remaining time, RoK played in defence to hold onto their lead.
"We played well in the first half and played attack-defence against our highly-appreciated rival. After our loss, the match changed, RoK defended rather well so our players couldn't level the score," said coach Williams.
Also losing to RoK, Myanmar and Hoang Anh Gia Lai (HAGL) will meet in the final group match tomorrow to earn a ticket to the semi-finals. The winner will company with RoK in the event's next stage.
"HAGL has many advantages, but we will play wholeheartedly and are determined to win the match," Williams added.