The Thai side, ranked 16th in the world, defeated Vietnam in straight sets, 25-17, 25-12 and 25-18. Three successive wins at Komaki were enough to earn the country a berth at the 2014 FIVB Women's Volleyball World Championship to be held in Italy next year.
The Thai team defeated Australia on Wednesday and Taiwan on Friday and has not lost a set so far in the competition.
The Thais will meet the host Japan, ranked No. 3 in the world, on Sunday but the result will be meaningless as its place in the final is already assured, with the maximum nine points from three matches.
"We are happy to go to Italy," said coach Kiattipong Radchatagriengkai after the match. "Now we can prepare for the finals."
Thailand will be one of the four teams representing Asia in the World Championship, which brings together the 24 best teams in the sport.
All five teams - Thailand, Japan, Australia, Taiwan and Vietnam - are playing a round-robin format in Komaki, north of Nagoya, with the two top going on to Italy in September 2014.
Russia was crowned the world's best in the competition hosted by Japan in 2010, edging Brazil in a five-set match.