Own goals by goalkeeper Chatchai Bootprom and Wanchai Jarunongkran plus a missed penalty by Kakana Khamyok undermined Thailand's hopes of overturning a 2-0 first-leg deficit despite strikes by Yotsakon Burapha and Wanchai.
The Thais struck first at My Dinh National Stadium when Yotsakon thumped a low strike past Patrik Le Giang, beating the Vietnam goalkeeper to his right.
Thailand squandered the opportunity to level the scores on aggregate four minutes into the second half when Kakana fired his penalty over the bar moments after being brought down in the area.
That miss proved pivotal as Vietnam scored in the 52nd minute.
Chatchai was the unsuspecting villain, the ball hitting the Thailand goalkeeper on the heel and crossing the line after Hoang Hen's first-time strike had clipped the base of the post and rolled along the goal-line.
Thailand gave themselves a glimmer of hope with six minutes remaining as Wanchai shot across Patrik and into the far corner.
But the chances of Thailand taking the game into extra time were extinguished deep into stoppage time when Wanchai thumped the ball into his own net, sealing Vietnam's fourth title win in the biennial competition's 30-year history.
With this victory, Vietnam not only successfully defended its title but also extended its unbeaten streak under head coach Kim Sang Sik to a record 26 matches.
Vietnam’s Nguyen Dinh Bac and Nguyen Xuan Son share the Golden Boot award with five goals each. Dinh Bac was also named the tournament’s MVP.
