Vien placed third in heat four with a time of 2min 13.9sec and ranked eighth among the finalists. Her coach, Dang Anh Tuan, said the qualifying round result showed that Vien had no chance to win a medal in the event.
"So we allowed her to withdraw, to enable her to remain in shape for the 200m backstroke and 800m freestyle competitions tomorrow," Tuan said.
The teenager on August 17 won a gold medal in the women's 200m individual medley category with a time of 2:12.66. This helped her become the first Vietnamese athlete to qualify for the Rio 2016 Olympics in Brazil.
The 13-day Games opened on August 16 with the participation of 3,800 athletes from 204 countries competing in 28 sports.