A last-gasp blunder by Thailand goalkeeper Kawin Thamsatchanan gifted Vietnam a 1-0 win in the King's Cup in Buri Ram Wednesday night.
Kawin failed to hold what looked to be a soft header by Vietnam's Nguyen Anh Duc as the ball went into his own net.
Park Hang-Seo's Vietnam will play in Saturday's title match against Curacao who beat India 3-1 earlier Wednesday at Chang Arena.
Thailand will meet India, who hammered the War Elephants 4-1 at the 2019 AFC Asian Cup, in the third-place play-off at the same venue on Saturday prior to the final.
Thailand striker Teerasil Dangda is likely to get his 100th cap in the match against India.
Coach Sirisak Yodyardthai did not use him in Wednesday's game as the veteran forward was not fully fit.
In Wednesday night's game, Southeast Asian champions Vietnam fielded a strong side led by Dang Van Lam, Nguyen Quang Hai and Nguyen Van Toan.
The hosts were without several key players including playmaker Chanathip Songkrasin.
Their strike force was led by Buriram United stars Supachai Chaided and Supachok Sarachart.
Thailand were the better side and had more scoring chances in the first half.
Supachai came close to opening the scoring in the 17th minute when his shot hit the post.
Nguyen Van Toan caused trouble for Thailand's defenders several times but failed to send the ball into the back of the net.
In the second half, Bui Tien Dung's hit the woodwork while Supachai had a couple of chances.
Supachai was replaced by 16-year-old Suphanat Muenta, another Buriram United player, in the 84th minute.
As the game seemed to be heading for a penalty shoot-out, Kawin's error handed Vietnam a ticket to the final.
Earlier, King's Cup debutants Curacao comfortably beat India 3-1.
Luis Bonavacia opened the scoring for the Concacaf side after 16 minutes and Elson Hooi made it 2-0 two minutes later.
Sunil Chhetri, who was making his 108th appearance for India, cut the deficit to 2-1 in the 30th minute.
Johan Bacuna restored a two-goal lead for Curacoa two minutes later.
It was Croatian Igor Stimac's first match as coach of India.