In India on Friday the Koreans dominated right from the start and took only four minutes to open the scoring through an accurate long shot by captain Ji So-Yun, who plays in England for Chelsea.
Three minutes later Choe Yuri went down the right flank and crossed it for Lee Geum-min to head in off defender Tran Thi Phuong Thao. It was counted as an own goal.
After conceding the two early goals, Vietnam played defensively and South Korea did not create any significant chance until the last 30 minutes of the game.
The Vietnamese began to tire, and South Korea increased the pressure.
In the 71st and 74th minutes goalkeeper Tran Thi Kim Thanh made two excellent saves but could not do much five minutes later when Ji So-yun scored from the penalty spot.
Defender Chuong Thi Kieu had handled the ball inside the box while trying to clear it.
South Korea kept pressing but could not score again, and the match ended 3-0.
In the other match in group C, Japan thrashed Myanmar 5-0 to go top.
Vietnam play Japan on Monday.