Sales were highest since May when over 43,800 units were sold, according to data from Vietnam Automobile Manufacturers Association (VAMA).
But it rose only 2% month-on-month, against 20% in July.
The most popular car sold in August was the MPV Mitsubishi Xpander at 2,842 units.
For the first eight months, 262,940 cars were sold in Vietnam, up 49.9% year-on-year.
Truong Hai Auto Corporation (Thaco) led in sales in the period at 93,347 units, up 54% year-on-year.
The three following brands, Toyota, Mitsubishi and Honda, all posted double-digit growth in sales.
Ford ranked fifth with a sales decline of 6.7%.