Pokklaw, who came on in the 66th minute, and beat Vietnam goalkeeper To Vinh Lol with a stunning long-range shot from just outside of the box with nine minutes remaining, sparking wild celebrations among the home crowd.
Another substitute Adisak Kraisorn, who came on in the 76th minute for Kroekrit Taweekarn, was lively in the short period of time he was on the pitch and twice came close to score for the home team.
A minute after he came on the lanky striker went close with a free header and again in stoppage time when he broke away down the left wing but his attempts were denied by To who had an impressive game.
Nguyen Minh Chau was sent off after receiving his second yellow for a foul on Sarach Yooyen in the 72nd minute.
"We definitely felt the pressure before we scored," said match winner Pokklaw after the match, "Vietnam defended very well and I'm glad I scored today and we got the three points."
Coach Kiatisak Senamuang said: "The Vietnamese played well in defence. They had many chances from playing a counter-attacking game but luckily our keeper Sinthaweechai [Hathairattanakul] made some good decisions and managed to get rid of all the danger.
"Our players played very well and created a lot of chances. The three points that we got today is very important for the team," he added.
Thai players endured some rough treatment from Vietnam, especially playmaker Chanathip Songkrasin, and the nervy match looked likely to end goalless despite Kiatisak's team dominating the possession and creating most of the attacks.
The Vietnamese defence looked shaky at times but the home players, especially striker Teerasil Dangda, wasted several good opportunities.
Teerasil, however, was the one who set up Pokklaw's winner. The Muang Thong United striker, who had a short spell in La Liga with Almeria, crossed the ball from the left to Pokklaw who fired a left-footed shot into the bottom of the right corner.
The other teams in Group F are Iraq, Taiwan and Indonesia. Thailand's next match is away to Taiwan on June 16.
The eight group winners and four best runners-up advance to Asia's final qualifying round for the 2018 World Cup.
Four Asian teams earn automatic spots at the Russia World Cup while a fifth team will play in an inter-confederation play-off.
Thailand have never reached the World Cup finals.
Sunday's match was originally scheduled for next month but moved forward because the date clashed with the SEA Games.
Read original story here.