Viencame in first in the women's 200m individual medley category, with atime of 2min 12.66sec, setting a new record for the Games.
The old record of 2:14.53 was set by Kaitlyn Jones of the US four year ago in Singapore.
Siobhan Bernad Haughey of Hong Kong took the silver, just 0.55sec behind, and Meghan Small of the US won the bronze medal.
Vien,17, earlier finished second (2:16.02) in the fourth heat of thequalifying round, then booked her place in the final, which saw herranked sixth among the eight best athletes.
However, theVietnamese surprised all with a strong splash in the final, creating alarger distance from her rivals after every 50m-lap.
Vien alsomade national history as the first-ever swimmer winning at the YouthOlympics, setting a new national and Southeast Asian Games' record.
"I am so happy. I just swam, swam, swam and I won. That's all," Vien was quoted as saying on nanjing2014.org.
Inthe upcoming days, Vien will compete in eight other events, among themthe 400m and 800m freestyle, which are her strongest events.
The Vietnam team also pocketed one silver medal in the men's 56kg category in weightlifting.
Nguyen Tran Anh Tuan failed to defend Vietnam's gold medal won in the last Games.
Hesuffered a stretched ligament prior to the competition, causing hispoor performance of 243kg. This placed him in second position, behindChinese Meng Cheng, who lifted 283kg.
With one gold and one silver from the first day of competition, Vietnam has successfully reached its target.
The13-day Games opened in wet Nanjing on August 16 with the participationof 3,800 athletes from 204 countries competing in 28 sports.-VNA