The latest victim was from the southern Binh Phuoc province. In addition, a 23-year-old man from the Central Highlands has tested positive for the H5N1 strain of the virus, but is still alive, an official from the Pasteur Institute in Ho Chi Minh City said.
Both men tested positive for the strain of avian influenza at the institute in the southern business capital of Phan Van Tu, a virologist at the institute said.
In human terms, Vietnam is the worst affected of 10 Asian countries tackling the disease. Thailand, where five people have died, is the only other country with confirmed human infections.
The World Health Organisation (WHO) has warned H5N1 could kill millions across the globe if it combines with a human influenza virus to create a new, highly contagious strain transmissible among humans.
"We are in a situation where people have become infected with H5N1 but we haven't crossed the threshold into this new threat level," Bob Dietz, the WHO's spokesman in Vietnam, said on Monday.
The UN health agency had said it was possible that two Vietnamese sisters who died on 23 January caught the disease from their brother, sparking fears that the virus had mutated into a far more lethal form.
But in a follow-up investigation the WHO said virologists in Hong Kong had failed to find any human genes in the virus samples taken from the sisters.
Thousands of birds have been
culled due to the spreading virus
This, it said, indicated that H5N1 "has not changed to a form easily transmitted from one person to another". But it did not categorically rule out human-to-human transmission.
Pascale Brudon, the WHO's representative to Vietnam, warned on Sunday that further human infections were inevitable.
"There is not a huge acceleration in the number of cases but we can expect to see more cases in different parts of the country," she said.
Of the 19 people confirmed to have been infected with H5N1, 13 have died and two others have made a complete recovery. Besides the two men, an eight-year-old girl and a 20-year-old woman remain in hospital.
The country's leader, Communist party chief Nong Duc Manh, has urged central and provincial authorities to ensure that bird flu is contained by the end of the month.
But disease-control experts say Vietnam has little chance of meeting its self-imposed deadline.
They say the further culling of poultry, improved hygiene and the mass vaccination of chickens to create buffer zones are essential to bring the disease under control.
More than 17 million birds have died or been killed across 57 of Vietnam's 64 provinces, but agriculture officials say they will not vaccinate poultry until the epidemic is over.