The number of cases increased 40.5 percent in the period from January to July, as compared with the same period last year, said the General Statistics Office (GSO).
The GSO did not give a death toll, but the deputy head of the country's preventive healthcare department, Nguyen Van Binh, told the communist party mouthpiece Nhan Dan, or People Daily, that 35 people had died.
The illness has spread rapidly in Vietnam's central and southern Mekong Delta provinces, Binh said.
The World Health Organisation (WHO) has expressed concerns that Asia could see a dengue epidemic this year on a par with that of 1998, when nearly 1,500 people died.
More than 1,000 people have been killed by dengue fever in Indonesia alone this year.
Dengue fever is a flu-like illness spread by the bite of an infected mosquito. Unlike the mosquitoes that cause malaria, those carrying dengue bite during the day.