The U.S. brand sold over 7,700 of the total of 9,500 vehicles in the segment, according to the Vietnam Automobile Manufacturers Association.
Its sales grew by 1.9% year-on-year.
Mitsubishi Triton was second, selling 991 units. It was followed by Toyota Hilux, which made a comeback with a nine-fold surge in sales to 576 units.
Isuzu D-Max and Mazda BT-50 followed in that order. The latter dropped out of the market in May.
Another truck sold in Vietnam is the Nissan Navara, but the company does not disclose sales figures.