The rangers said the monkeys belong to the southern pig-tailed macaque species, which has been listed by the International Union for Conservation of Nature as vulnerable. In Vietnam, the species is protected.
The monkeys weigh a total of 35 kilograms. They are weak, after being held in bags for a long time.
Huynh Minh Tien, 22, has admitted hunting the monkeys and planned to sell them in his hometown Phan Thiet in the south.
He is facing cash fines for violating wildlife protection rules.
The southern pig-tailed macaque is threatened by loss of habitat, particularly to oil palm plantations in Malaysia and Indonesia.
The species is also shot as a crop pest and hunted for food.