Having lost 2-1 to the Vietnamese in the semi-final first leg on Sunday, the Malaysians shocked the hosts 4-2 in the second leg to win 5-4 on aggregate.
In the two-legged final, Thailand, who beat Malaysia 3-2 in the group stage, will host Malaysia at Bangkok's Rajamangala National Stadium on Dec 17 and go to Kuala Lumpur on Dec 20.
Thailand have won the Asean title three times and Malaysia once.
It will be their second meeting in the final with the Thais beating the Malaysians in the inaugural tournament in 1996.
Thailand are chasing a record equalling fourth title in the biennial championship.
The Thais last won the title in 2002 and lost to Singapore in the previous final in 2012 as the Lions claimed their fourth crown.
Thailand reached the 2014 final after hammering the Philippines 3-0 at Rajamangala on Wednesday night for a 3-0 aggregate win.
Thailand coach Kiatisak Senamuang said yesterday the players will train in Bangkok and can leave the training camp only for really important matters.
"We want to win back the title to end a 12-year drought. We are only two matches away from regaining the title so we will train hard for the final," said Kiatisak who won the title three times as a player.
"We have improved since the group stage. Our defence is now stronger and our midfielders are better."
Tickets for the Dec 17 match start from 150 baht and will be available at Thai Ticket Major on Friday (December 12). Bangkok Post