It was a close final, but the host came out on top with a goal scored by Nham Manh Dung in the 83rd minute.
"SEA Games mean a lot to me," Park said after the game.
"Two years ago I coached the U23 team to the first men's football gold for Vietnam in 60 years. This time the SEA Games are being hosted in Vietnam, and I felt responsible because the expectations from the fans are high.
"This is also my final game with the U23 team so I was under severe stress. I want to thank all players and the fans."
Park started coaching the U23 team in 2018 and took them all the way to the final of AFC U23 Championship in China.
He also led them to the semifinals of Asian Games and won two more SEA Games gold medals. But after the current SEA Games he will stop coaching the U23 team to focus on the senior team, and his next target is the 2022 AFF Cup, which was claimed by Vietnam in 2018 and by Thailand in 2020.
Asked about the rivalry between Vietnam and Thailand, he said: "Rivalries happen everywhere in football, and it's good for the development of the region."
In 10 days' time Vietnam will play in the U23 Asian Cup and meet Thailand again in the opening game.
Replacing Park as coach will be Gong Oh-kyun, another South Korean.
"Each coach has a different way of managing and operating the team. I don't know how the team will play with the new coach. But I think young players are not having many opportunities in domestic competitions. So the players who played in recent U23 tournaments will continue to be the core."
He said he would share with Gong his thoughts about the culture and pressure from fans and the media in Vietnam.