A 20-year-old man from a province neighbouring the capital Hanoi was killed by the H5N1 strain a week ago, the communist government said, and four more people are known to have been infected with the virus since last month.
The country's first bird flu death since October 2005 comes as avian influenza has made a strong resurgence across Vietnam, hitting poultry flocks in over 100 outbreaks across 18 of the country's 64 provinces and municipalities.
"This is not going to go away," said WHO's Vietnam communications officer Dida Connor, speaking before news of the human death. "There is a sense of complacency which is potentially catastrophic if it was to increase."
The human fatality, which brought the country's bird flu death toll to 43, followed several avian influenza cases across Southeast Asia last week.
On Thursday, Indonesia said a 29-year-old man had died of bird flu, bringing the death toll in the country worst hit by the virus to 80.
In Malaysia five people were quarantined with suspected bird flu, while Myanmar reported a fresh poultry outbreak.
Vietnam's unusual summertime outbreak, concentrated in the densely populated northern Red River delta region, follows the ending in March of a two-year ban on duck hatching that has triggered a surge in production.
Nationwide vaccination campaigns -- widely hailed as a model that other countries have sought to emulate -- have become increasingly spotty, Vietnamese and international animal health officials have warned.
"We've had bird flu for four years," said the UN Food and Agriculture Organisation's Vietnam bird flu specialist Jeffrey Gilbert. "Everyone's tired of it... but it's going on, and we are increasingly challenged to get these messages across to people that the risk hasn't gone away."
"People get very bored very quickly," he added.
"We can get the farmer to come in once or twice, but the third time he may see that it's not much of a priority anymore. He may not bother, or he may bring in 50 ducks and not bother to notify the authorities that there's another 150 (needing vaccination) still in the field."
Ducks and other waterfowl, which can carry and spread the virus without showing symptoms themselves, have been the major source of infections in the latest wave of outbreaks, Vietnam's fifth since 2003.
Flocks of ducks roving across rice paddies and ponds are a traditional, iconic feature of the Vietnamese countryside. After the lifting of the ban, they have come back in great numbers following the northern rice harvest.
Prime Minister Nguyen Tan Dung last week ordered the agriculture ministry to map out immediate and long-term action plans to fight bird flu, concentrating on vaccinating Vietnam's 70 million ducks and culling all sick animals.
Experts warn that H5N1 survives both in the respiratory system and the gastro-intestinal tract of birds -- meaning farms, markets and slaughterhouses have to be vigilant about hygiene and biosecurity.
Vietnam, once the country worst affected by H5N1, was hit by three waves of avian influenza in 2004 and 2005, when 42 people were killed and authorities culled more than 50 million head of poultry.
No new outbreaks were reported for 12 months, until fresh infections last December led authorities to cull 45,000 poultry. Sporadic outbreaks have been reported since February, but cases picked up sharply in May.
Worldwide, bird flu has killed 191 people out of 313 infected patients, says the WHO, which has not yet confirmed Vietnam's latest human cases.
Experts fear the death toll would rise sharply if the virus were to mutate and become easily transmitted between humans. A flu pandemic in 1918, just after the end of World War I, killed 20 million people worldwide.