Tuyen Quang General Hospital said one of the victims it admitted died on Tuesday afternoon.
The bus was running on National Highway 2 at about 1:40 a.m. when it collided with a container truck going in the opposite direction.
According to initial information, five people were confirmed dead on the spot.
Eight others were seriously injured and taken to a nearby hospital for emergency.
At the scene, the container truck overturned and debris was scattered on the road.
Initial investigation found the truck's driver had lost control at a turn, swerved to the left and hit the bus heads-on. Both drivers tested negative for alcohol.
The truck driver was arrested later on Tuesday facing charges for violating traffic regulations.