The bus was carrying 42 foreigners and two Vietnamese when it crashed into a seven-seat car and a motorbike before flipping at around 6 p.m. Friday on National Highway 6 in Hoa Binh's Mai Chau District, around four-hour drive from Hanoi.
Police and local residents had to break the bus's windows to rescue the passengers one by one. The crash left 20 people injured - 15 foreigners and two Vietnamese on the bus, and three people on the car, who were then taken to hospitals in Hoa Binh and Hanoi.
The bus was lying on its side on the sidewalk after the crash, around 10 m from the car, while the motorbike ended up in a bush nearby. All three vehicles were damaged and debris was scattered across a road area.
Initial investigation found the bus driver had failed to slow down when driving down a slope and turning.
The bus was carrying tourists from different countries, including Italy, the Philippines and Ukraine.