The driver of the bus, carrying about 37 people, was travelling about 70km per hour when it swerved into the path of an oncoming vehicle to avoid a parked car and fell into a wooded area.
The passengers said they were fast asleep when the accident occurred.
"The only door of the bus was blocked. Rescuers had to smash the windows to free the passengers", said Nguyen Thi Kim Anh.
At least 27 survivors were freed from the wreckage and boarded another bus to continue their journey to Hue city.
The accident happened at around 3am on March 24 in the Dai Quang commune of Dai Loc district and difficult access to the site hampered the rescue effort, a local official said.