The typhoon, which made landfall late Friday, was downgraded to a tropical depression Saturday but continued to dump water on already flooded parts of the country.
A taxi driver was killed when high winds caused a tree to fall on his car in Hanoi, while two others died from electric shock after a cable was felled in northern Son La city, according to an update from authorities.
In Bac Giang province a 46-year-old woman died after a hill collapsed burying her house in the middle of the night, VNExpress news site reported.
Earlier more than 11,000 boats, including several hundred used by tourists at the UNESCO world heritage site Halong Bay, were ordered to stay close to the shore.
The Vietnamese army put 20,000 soldiers backed by helicopters, rescue boats and canoes on standby to handle any incidents.
Kai-Tak swept across Philippines’ main island of Luzon, dumping heavy rain on the Cagayan basin and other areas in the north, killing four people.