The most recent death was reported to the Ministry of Health on April 17, WHO said Thursday on its Web site, providing no other details.
WHO officials in Hanoi typically receive official word of bird flu deaths days after the fact and details are often limited.
The global body said it had asked Vietnamese health officials to provide more individual data on cases.
"Rapid field investigation of new cases, especially when these occur in clusters, remains essential to assess possible changes in transmission patterns that could indicate pandemic potential of the virus," it said.
On Thursday, the WHO in Geneva again warned that bird flu poses a great potential threat to humans if it evolves into a virus that can easily spread from person to person. There is no evidence a change has occurred and most cases have been traced back to contact with sick birds.
Separately, authorities in the southern commercial hub of Ho Chi Minh City have advised people to not raise poultry from November this year to February 2006 to reduce the risk of the virus spreading, said Phan Xuan Thao, deputy director of the city's animal health department.
"We have highest risks of the bird flu outbreaks recurring at this period of time," he said, adding that there would be an increase in poultry sales ahead of the Lunar New Year festival which falls in late January, when many people travel to visit their families.
Ho Chi Minh city, home to eight million people, has not reported any human cases of bird flu since Nov. 2004.