Bualoi made landfall on Monday in northern central Vietnam, bringing huge sea swells, strong winds and downpours. Last week, the typhoon had killed at least 10 people in the Philippines.
The government said 88 people had also been injured, more than 100,000 houses were damaged, mostly in Nghe An and Ha Tinh provinces, and more than 10,000 hectares of rice and crops had been inundated.
Rainfall in several parts of Vietnam had exceeded 300mm over the past 24 hours, the national weather agency said as it warned of a risk of landslides and flash flooding.
In the capital Hanoi, streets were heavily flooded as persistent downpours were accompanied by thunder and lightning, paralysing traffic in many downtown areas.
"It doesn't rain, it pours," said 49-year-old Hanoi resident Hoang Quoc Uy.
"Water is flowing into my living room... I've never seen anything like this before."
As of Tuesday, several villages in northern central Vietnam remained flooded with no traffic access or power, state media reported.
Floodwaters rose to close to the roof level of houses in several villages in Nghe An province, according to footage on state broadcaster VTV.
"This is my house, the roof above had been blown away by the typhoon wind and it is now half a metre deep in flood water," 56-year-old Nghe An resident Ngo Thi Loan told Reuters.
"All of my belongings have been damaged, all gone," Loan said.
With a long coastline facing the South China Sea, Vietnam is prone to typhoons that often also bring heavy rains that cause severe flooding.
