Although Nguyễn Tiến Linh's header in the first half gave Việt Nam the lead, calamitous defending in the second half led to Thailand scoring two away goals courtesy of Poramet Arjvirai and Sarach Yooyen, though the Golden Star Warriors were redeemed through Vũ Văn Thanh's equaliser from 25 metres.
Only one change was made from Park Hang-seo's last match in Mỹ Đình, with Đỗ Duy Mạnh replacing Nguyễn Thành Chung as the right centre-back.
The match started with the men in red taking the initiative, putting the Thai defence under huge pressure.
Phạm Tuấn Hải was gifted with a golden chance when he was through on goal in the 9th minute, but his shot went just wide.
Five minutes later, Thailand replied when Adisak Kraisorn was put through on goal, but the centre forward could not beat Đặng Văn Lâm in a one-on-one.
With Việt Nam piling on the pressure, the sold-out crowd at Mỹ Đình did not have to wait long for the men in red's opener.
In the 23rd minute, Quế Ngọc Hải, on a rare occasion pushing forward, crossed in for Nguyễn Tiến Linh, whose low header beat Thai keeper Kampol Pathomakkakul and bounced into the net.
This was Linh's sixth goal for Việt Nam in the 2022 AFF Cup campaign, making him the current top scorer of the tournament, one goal ahead of Thailand's veteran Teerasil Dangda, who did not feature in the match because of an injury.
Seven minutes after his opener, Linh put the ball into Thailand's net again, but the referee disallowed the goal due to a foul on Kampol by Linh.
Thailand started to reclaim control of the ball, putting the Vietnamese defence on high alert in the final minutes. Theerathon Bunmathan, captain of the War Elephants, was the mojo behind every Thai attack. The left-back even hit Văn Lâm's woodwork with a free-kick from close range in the last moments of the first half.
If Theerathon's free-kick was a grave warning for Việt Nam, Park's defenders must not have paid close attention. The 32-year-old defender, who spent five years playing in Japan's J.League 1, struck a long ball from within his own half that broke Việt Nam's offside trap in the 48th minute.
The ball found Poramet Arjvirai, who was through on goal and finished with a vicious strike into the far corner of Văn Lâm's net, scoring the away goal that Thailand needed.
Poramet's goal was the first goal Việt Nam conceded this tournament, but the horror of Friday the 13th did not end there for the men in red.
In the 63rd minute, Theerathon pushed forward in midfield and put a De Bruyne-esque through ball, breaking Việt Nam's defence for the second time. Sarach Yooyen was slightly onside and beat Văn Lâm in a one-on-one, nudging Thailand ahead.
With nothing to lose, Việt Nam used up all five changes to fix their flawed defence and bolster their attacking power.
One of those changes made a surprising difference in the match's final minutes.
It seemed that Việt Nam's story this tournament was about disappointing players shining when the team needed them the most. After Tiến Linh was Văn Thanh, who came on as a replacement for Tấn Tài in the 85th minute. Pundits heavily lambasted Thanh for having zero impact from the right wing throughout the tournament.
But only three minutes after he was brought on, Thanh made a deep and sudden impact on Thailand's net with a 25-metre screamer that sent Mỹ Đình wild.
Việt Nam continued to push forward after Thanh's equaliser, looking for a win, but Thailand's defence stayed strong. The Thais could even have won it in Mỹ Đình had it not been for Văn Lâm's last-minute heroics.
With this result, Việt Nam are at a disadvantage on away goals before entering the second leg in Thailand on Monday. Having drawn 2-2 at home, the Golden Star Warriors needed a win on Thai soil, or a draw 3-3 or more to win their third AFF Cup.
A 2-2 draw in Thailand will take the game to extra time and penalties if needed. /.