Floodwaters reached the tops of cars and rooftops in areas of Thai Nguyen city, about 80 kilometres (50 miles) north of the capital Hanoi, with many left stuck at home and others forced to flee.
The environment ministry said eight people were killed in flash floods and landslides in Vietnam's mountainous north since Monday, and five others were missing.
"I have never witnessed such a terrible flood since I was born 60 years ago," Nguyen Van Nguyen told AFP from his three-storey house in Thai Nguyen province.
"There has never been flooding here in my street but now my ground floor is all submerged."
The military said it had mobilised 30,000 personnel and thousands of boats to join rescue and relief efforts.
It used three helicopters to drop more than four tonnes of water, instant noodles, dry cake, milk and lifejackets to people in flooded parts of Lang Son province.
From late Tuesday to Wednesday, the country recorded the highest flood levels on three rivers in the north in nearly 40 years, state media reported.
The high-water mark of Trung river in Lang Son, bordering China, was forecast to peak at nearly two metres (6.5 feet) above the record, Vietnam state television said.
The Bang and Thuong rivers also surpassed levels not seen since 1986.
By Wednesday morning, the weather bureau said the Cau river, running across Thai Nguyen city, was more than a metre higher than the previous record level -- when Typhoon Yagi devastated the country in September last year.
- 'Totally flooded' -
Overnight Tuesday and Wednesday, social media users posted pleas for help as their relatives and friends were left stranded with no electricity and few provisions in the provinces of Thai Nguyen, Cao Bang and Lang Son.
"Our ground floor (in Thai Nguyen province) was totally flooded. My parents and five kids were stuck, with not enough food and water. No communication since late Tuesday. They need urgent help," Thoan Vu posted online alongside hundreds of similar pleas.
More than 200 families living in Lang Son province, downstream from a dam that burst Tuesday afternoon, were earlier evacuated to shelters, the environment ministry said.
The cracked dam, part of the reservoir for the Bac Khe 1 hydropower plant, caused about $1.9 million in estimated economic losses, the ministry said in a statement.
"The reservoir of the small hydropower plant has a capacity of four million cubic metres of water... so together with heavy rains, neighbouring communes may be inundated, but no flash floods were forecast," it said.
Scientists say human-driven climate change is making extreme weather events like typhoons more deadly and destructive.
The floods followed heavy rain from Typhoon Matmo, which weakened on Monday while approaching Vietnam but hit the north hard.
Matmo landed only a week after Typhoon Bualoi triggered widespread flooding, killing at least 56 people and causing economic losses estimated at more than $710 million.
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