The bus was travelling from Buon Ma Thuot city in the central highlands to southern Ho Chi Minh City, a policeman in Krong Pak district where the accident took place late Thursday said on condition of anonymity.
"We are investigating the cause of the accident and identifying the victims," he told AFP on Friday.
The wreckage of the bus was raised from the Serepok river bed early Friday, he said.
A total of 54 people, including the driver, were believed to be on the bus.
Road accidents are common in communist Vietnam and kill about 35 people every day. Narrow highways, poorly maintained vehicles and drivers' disregard for road safety and traffic rules are blamed for most fatalities.