ASEAN Championship
Despite Thailand dominating the tie, Vietnam were clinical in their finishing and will head to Sunday's return leg confident of winning a third title.
Both sides had chances to open the scoring but the breakthrough - for Vietnam - came in the 59th minute when a pinpoint long ball from Nguyen Quang Hai found Van Thanh on the flank, whose perfectly placed header set up Rafaelson for a clinical close-range finish.
Rafaelson doubled Vietnam's lead in the 73rd minute when he robbed a Thai player of the ball, drove forward and unleashed a strike that left goalkeeper Patiwat Khammai rooted.
Thailand, however, gave themselves hope of an eighth title through Chalermsak Aukkee’s 83rd minute header.
Photos: Vietnam Football Federation