The collapse occurred at kilometre marker 76+300, where a section of the road was washed away, cutting traffic between the two provinces.
A separate landslide also blocked National Highway 279 near Muong Than Commune after a large volume of soil and rock fell onto the road.
Authorities said it was too early to estimate when either route would reopen because of the continuing risk of further landslides.
Police, local authorities and road maintenance crews have cleared debris where possible, installed warning signs and diverted traffic.
Motorists have been advised to avoid the affected section of National Highway 279 until further notice.
The disruption follows days of heavy rain across northern Vietnam, where forecasters have warned of continued downpours through July 18, raising the risk of flash floods and landslides.