Two passenger buses collided on an expressway that connects Hanoi and Sa Pa on Monday afternoon, killing both driver assistants and leaving 12 people seriously injured, local media reported.
The head of one bus was smashed after it hit the other from behind, pushing the latter off the road.
Both were running from My Dinh terminal in Hanoi and had traveled 70 kilometers.
Police are investigating the cause of the crash and emergency responders are still helping the victims.
The Noi Bai-Lao Cai expressway, currently the longest in Vietnam, has recorded many accidents since it opened to traffic in September last year. Lao Cai is most famous for its charming town Sa Pa.
The latest crash was a head-on collision between a car and a passenger bus on December 18 that killed the car driver and injured three others in the same family.
The US$1.5-billion expressway has four lanes and only allows a speed of 80 kilometers per hour now, even though the designed speed is 120 kilometers per hour.
UPDATE: Initial investigation found the bus running behind might have lost control after a tire broke. Tuoi Tre newspaper reported on Tuesday that 22 people were injured in total.