The bus was traveling on National Highway 2C at around 5 a.m. when it mowed down almost a dozen people attending the funeral.
The deadly accident happened at an intersection in Yen Lac District, 50 km northwest of Hanoi.
Local authorities said four people died on the spot and three others succumbed to injuries in hospital. Another person is under treatment in critical conditions.
At the time of the accident, it was misty, according to eyewitnesses.
The police are investigating the cause of the accident.
Road crashes are a leading cause of deaths in Vietnam, with one person dying almost every hour. A total of more than 18,720 accidents occurred in 2018, killing 8,244 people and injuring nearly 14,800, according to the National Traffic Safety Committee.
Earlier this month eight people lost their lives while walking along a highway in the northern province of Hai Duong after a truck mowed them down. Last January four people died and 16 others were injured in the southern Long An Province after a truck slammed into motorbikes waiting at a traffic light.
In both cases, the drivers have been found to be high on drugs.