The accident in Binh Thuan province left at least 36 others seriously injured, Tuoi Tre newspaper said in an online report.
The two buses were believed to have collided with each other before hitting the truck, with all three vehicles later bursting into flames.
"The two buses and part of the truck were burned out," the report said.
Road accidents are common in communist Vietnam, killing an average of 35 people every day.
Narrow highways, poorly maintained vehicles and drivers' disregard for road safety and traffic rules are blamed for most fatalities.