A first-half brace from captain fantastic Chanathip Songkrasin means the five time champions go into the return leg on Sunday as favourites to progress to the final.
The result was a fair reflection of a tough game that the War Eelphants managed to control against the defending champions. Four yellow cards were administered to Thai players during the match and three to Vietnamese players as things started to get heated in the second-half.
Should they advance to the final, Thailand will be looking for a record-breaking sixth title.
They will face either Indonesia or Singapore in the showcase event on Jan 1. The teams drew 1-1 on Wednesday and will face one another again tomorrow to determine who wins and progresses to the final.