The Vietnamese side got ahead 26-24 in the first set, but were not strong enough to maintain their lead in the second, in which they lost 18-25.
The third set saw equal score for both sides. But the Thai team were more aggressive in scoring the final points, and won 25-23.
Throughout the four sets, although the Vietnamese team gave it their all, Thailand proved to be the better team. Their powerful volleys pushed Vietnam into defence, before winning 25-20.
With two wins at the Southeastern Asian zone, Thailand grabbed the only berth in the next qualifier.
Earlier, Thailand beat Myanmar 27-25, 25-17, 25-22 on October 7. Vietnam also secured a win of 25-20, 25-13, 25-18 over Myanmar on October 8.