Vietnam, Japan and Saudi Arabia all finished with six points in group D, but Japan topped with a goal difference of +1 and Vietnam were second with 0. Saudi had a -1 difference and were eliminated.
If Vietnam had lost 0-1 to Japan, they would have finished top.
But they focused on defending, thus playing into their opponent's hands, and Kazuya Shimizu scored from individual efforts in the fifth and 35th minutes. Vietnam attacked desperately in the final minutes but could not score.
Vietnam, coached by Diego Giustozzi, will play Iran, the best futsal team in Asia at the moment and winner of 12 of the 15 previous editions, on Tuesday.