More than 500 millimeters of rain have fallen on the city in the past three days.
At least 18 people have died in the capital alone, with many streets flooded, water supplies contaminated with sewage and schools and business forced to close.
Rescue teams used trucks, boats, makeshift rafts and horse carts to carry people and motorcycles along the flooded city roads.
Across the country many more people are reported as missing after being swept away by floodwaters.
|Hanoi was under up to three meters of water after days of heavy rains [Reuters]
Vietnamese television on Sunday night quoted Nguyen The Thao, Hanoi's mayor, as saying that it would take the city four or five days to pump excess water into the Red River, or longer if it continues to rain.
The national committee for flood and storm control said on its website that more 240,000 hectares of rice and vegetables have been destroyed and about 170 kilometers of rural roads have been damaged.
"I have been stuck in my house for the past three days," said Nguyen Manh Hung, a businessman in southern Hanoi where floodwaters reached chest level.
"It's unbelievable to see people navigating the street in boats and by horse-drawn carriages."
Elsewhere in Asia days of heavy rains in southwestern China triggered a mudslide that killed at least 20 people and left 42 others missing, state media has reported.
A torrent of mud and rocks hit the city of Chuxiong in Yunnan province on Sunday according to the official Xinhua news agency.
The flooding and mudlside left more than 1,000 houses collapsed or damaged.