Tran Quy, director of Bach Mai hospital in Hanoi, said the latest Vietnamese case of the disease bore all the typical symptoms of bird flu.
"He has a high fever, difficulty breathing, coughing and the X-ray of his lung was completely white," he said.
The man's relatives said he had helped prepare chickens for a wedding reception before falling ill and being warded last week.
Samples of the virus will be shipped to a World Health Organisation (WHO) lab for analysis and reconfirmation.
Vietnam had been hailed a success story for beating back the virus that began ravaging Asian poultry stocks in late 2003.
A nationwide mass poultry vaccination program, coupled with strong political will, brought the virus under control after it killed 42 people in the country.
No poultry outbreaks were reported in 2006, but the virus flared again early this year.
Virus sharing deal
Meanwhile the WHO has said it has struck a framework deal to ensure states share samples of the H5N1 virus and assuring countries that any vaccines produced will be fairly priced.
The agreement was "a big step forward in virus sharing" the organisation said, as it works to develop an up to date vaccine against the virus.
Indonesia only recently resumed sharing samples of the virus with the WHO after it was assured that pharmaceutical companies would not price any vaccine beyond the country's reach.
Scientists fear the H5N1 virus could mutate into a strain that spreads easily among people, triggering a pandemic that would sweep the globe.
The virus has killed up to 185 people worldwide, according to the WHO.