NisaRomyen scored a double for the visitors, who will be the first Southeast Asian team to take part in the World Cup later this year in Canada.
Romyen netted her first goal in the 52nd minute after passing Vietnamese defender Nguyen HaiHoa in the box and beating goalkeeper Dang ThiKieu Trinh.
Her second goal came in the second minute of additional time as she left all Vietnamese defenders behind and chipped the ball over custodian Trinh's head. She is the tournament's leading goal scorer thus far, finding the opponent's net eight times.
Viet Nam's only goal was the game's opener in the 30th minute via a strong header from Nguyen Thi Lieu that Thai goalie WarapornBoonsing had no chance to save.
Meanwhile, striker NawArloWerPhaw's solitary goal in the second half saw Myanmar defeat Australia's U20 team and book a place in the finals.
"We know that the Australians were tactically stronger but I thought that our girls managed to raise the level of their game," said Myanmar coach ThetThet Win on aseanfootball.org.
"Now that we are in the final, it really does not matter who we play against. The target is to win the cup, so we must be able to beat any one."
The Australian women, who had been so dominant in the group matches seemed to be a little off colour on the day even though they tried hard through the wily Alexandra Chidiac.
But it was Myanmar who controlled most of the pace with WerPhaw and Yee YeeOo in the forefront.
"I have no complaints," Australian coach Ante Juric said later. "They are still young girls and I thought that they played really well. They never give up."
VNS