Captain Chanathip Songkrasin opened the scoring on 14 minutes and it was the Thai striker who scored another goal nine minutes later, extending the lead.
Vietnamese players carved out a number of scoring chances, but they failed in the end.
Thai striker Chanathip also failed to complete a hat trick from a penalty kick when the match had several minutes to go, but he was refused by Vietnamese goalkeeper Nguyen Manh.
The loss put Vietnam at a disadvantage when both teams meet again on December 26. The defeat also ended coach Park Hang Seo’s 35-match unbeaten streak against Southeast Asian teams.
Meanwhile, coach Alexandre Polking of Thailand said he was satisfied with his players’ performance.