The truck, driven by 32-year-old man Vo Duy Do, was heading south from the province's capital Tuy Hoa when it hit two motorbikes and one electric bike, said Vo Dinh Tien, chairman of Dong Hoa District, where the accident happened.
One motorbike was dragged by the truck for several meters and badly crushed.
The accident killed Nguyen Thi Bao Linh, 40, and her 5-year-old daughter in one motorbike, a 5-year-old boy on the other motorbike, and a 14-year-old girl who was on the electric bike, Tien said.
Three family members of the boy are still undergoing treatment.
Investigation found Do was driving under the influence of alcohol, and he did not have a driver's license. He has been held for investigation.
Road accidents are a leading cause of death in Vietnam. In the first 11 months this year, traffic accidents killed 6,975 people and injured 12,144 others, according to the National Traffic Safety Committee.