They said Nguyen Duc Trung, 31, drove a car to a villa compound in Long Bien District on Monday evening and accosted the boy, who was riding a bicycle there.
He opened the door to block the boy's path and quickly got out, bundled him in the car and drove away.
He made the boy give his parents’ numbers, and used unregistered sim cards to call them to demand VND15 billion.
The boy’s family contacted the police, but by night had got VND13 billion ready to pay the ransom.
Early on Tuesday Trung was arrested at an industrial park in Ha Nam Province’s Duy Tien District, 60 km south of Hanoi, where he was hiding with the boy.
The police said they had tracked Trung down and secretly followed him as he traveled from Hanoi to Hung Yen and Bac Giang provinces before stopping in Ha Nam.
When he was taken in, Trung shot at the police with rubber bullets, injuring an officer in the thigh.
He has since told the police he was deep in debt and therefore came up with the plan to blackmail the boy’s parents.
It is not yet clear whether he had learned about the boy's family before, or just acted randomly.
The boy has been safely returned to his family.