She beat 190 competitors to take the lead with a time of 2:16.06.
In the women's 100m backstroke, the 18-year-old swimmer also won a silver medal with a time of 1:04.07.
TheVietnam Aquatic Sports Association said reigning Youth Olympicschampion Vien has been named among those who would be awardedscholarships for receiving training support from the United StatesSwimming Federation for the 2016 Summer Olympics in Rio de Janeiro inBrazil.
Earlier, Military team's Vien qualified to compete in the women's 400m medley and 200m backstroke events at Rio.
Bornin the Mekong Delta city of Can Tho, Vien is now working with AnthonyTeeters who guided Ryan Lochte to achieve success, in Episcopal club inthe United States. She also spent two years training in Saint AugustineClub in Florida.
Vietnamese coach Dang Anh Tuan said Vien wasamong the world's top 20 female 400m medley swimmers and was playing foran American club, two conditions that helped her qualify to receivesupport from the United States Swimming Federation.
At 2014's event, Vien won three gold medals in the women's 200m medley, 400m medley and 200m backstroke events.
Alsoin that year, Vien won two Asian Games bronze medals that wereVietnam's first ever at the Games. She also set seven records atthe ASEAN Swimming Championships, and dominated the national tournamentwith 18 titles and 14 records.
Vien has been voted the best athlete of the year for the second time in a row.-VNA