A leader of Dong Loc Commune said the child was suspected to have been forgotten in the vehicle by her father while he was at work.
According to local officials, the man drove his two daughters on Thursday morning, dropping the older child at an elementary school but not taking the younger one to nursery.
He then parked the car at his workplace, locked it, and went inside, leaving the child in the back seat.
The girl was found later that afternoon by a colleague and taken to hospital but she did not survive, authorities said.
Nghe An Province has experienced extreme heat in recent days, with temperatures exceeding 38 degrees Celsius on Thursday.
Local officials have visited the family, and the case is under review.
Similar incidents have occurred in Vietnam, including the deaths of a six-year-old boy in Hanoi in 2019 and a five-year-old boy in Thai Binh Province in 2024 after being left in vehicles.
