Japan played attack since the first minutes of the match. They scored three goals in the first half thanks to Chikage Kichibayashi in the fifth minute, Maiko Higashiyama (10th minute) and Akari Takao (12th minute).
Futsal is a form of indoor football played on a hard surface with five players per side.
After the break, Kichibayashi scored her second goal at the match, Vietnam had two goals to narrow the gap in the 34th and 39th minute from Nguyen Thi Chau.
"For us, it was very important to win and we did it. In the first half we were calm, defended well, and played our game. In the beginning of the second half, I tried to rotate some players but our opponents pressed us well and we gave two goals away," said Japan's coach Masaaki Arihara on the-afc.com.
Vietnam's coach Truong Quoc Tuan said his team did not play well in the first half but they improved in the second period. If they didn't miss a few chances they had in the second half, the result would have been different.
"Tomorrow [Tuesday] we must win to stay in with a chance but after today's very hard match we have to make sure we recover well first," Tuan said.
Earlier, Thailand beat China 3-1.
Today, Vietnam face China and Thailand play Japan, who were Asian Indoor Games gold medalists in 2007, 2009 and 2013.
A total of eight teams are competing in this championship. They were drawn into two groups of four teams. The group winners and runners-up advance to the semi-finals.
Vietnam are in Group B with Japan, China and Thailand, while Group A consists of Iran, Malaysia, Uzbekistan and Hong Kong.
