Test results showed the birds were infected with the lethal H5N1 bird flu virus strain and died on a farm in Ben Tre province last week, the Department of Animal Health said in the report.
Authorities slaughtered all other birds on the farm and disinfected the area, it said.
A bird flu outbreak was reported in the northern province of Ha Nam on Wednesday. So far the virus has spread to six provinces across the country, and authorities called for improved efforts to prevent a pandemic before the winter season when the virus typically flares.
Bird flu, which ravaged farms across Asia in late 2003, has killed at least 205 people worldwide including 46 in Vietnam, according to the World Health Organization.
The virus remains difficult for people to catch, with most human infections resulting from contact with sick birds. But experts fear the virus could mutate into a deadly form passed between humans resulting in a pandemic that could kill millions.