The upgrade, which will double capacity to more than 5,000 seats a week, reflects rising traffic demand on the route.
The new flight will depart Bangkok's Suvarnabhumi for Tan Son Nhat airport at 4.35pm, said an airline official.
The existing Ho Chi Minh City flight, which takes 90 minutes, leaves Bangkok at 1.50pm.
The launch of the additional service on the route comes as the airline is due to take delivery of more brand-new Airbus 320 jets.
VietJet operates 27 domestic and international routes with a fleet of 15 single-aisle A320s.
Its domestic network covers Ho Chi Minh City, Hanoi, Haiphong and Danang, plus Nha Trang, Vinh, Hue, Phu Quoc, Dalat, Buon Ma Thuot, Quy Nhon and Can Tho.
Internationally it connects Vietnam with Singapore, Thailand and Incheon, South Korea.
The airline took to the skies in December 2011 and has grown to become Vietnam's second largest after flag carrier Vietnam Airlines.
According to an industry estimate, VietJet has a 26% share of seat capacity in Vietnam's domestic market compared with 60% for Vietnam Airlines.
In June 2013, VietJet signed a basic agreement with Thai commuter airline Kan Air to set up a Thai offshoot, Thai VietJet Air.
The joint venture is expected to begin serving Chiang Mai and Phuket from Suvarnabhumi airport with the deployment of two or three A320s.
Thai VietJet Air, to be 51% owned by Kan Air and 49% by VietJet, has yet to announce its launch date, which was earlier tipped for this year's fourth quarter.