The Can Tho province-born swimmer left all rivals behind to top the category, her strong point, with a time of 2:15.09min.
The first runner-up was Sarisa Suwannachet of Thailand whofinished 2:17.28. The second was Lee Hee-eun of South Korea , at2:17.93.
It was Vien's second gold and third medal from Nanjing City .
On August 21, she earned a gold in the 200m backstroke and a silver in the 100m backstroke one day earlier.
The Olympian Vien will next compete at the 50m backstroke event on August 23.
Vien's teammate Tran Duy Khoi later grabbed a bronze in the boys' 200m individual medley.
The Vietnamese team has strongly improved their position in theranking. With three gold, four silver and two bronze medals, the teamnow ranks eighth on the medal tally.
China hasno rivals for top place. They have 37 golds, 21 silvers and 21 bronzes.The Republic of Korea is second while Thailand comes third.-VNA