Over 20,700 units were sold by members of the Vietnam Automobile Manufacturers Association (VAMA) last month, a 53% drop from May last year.
Sales reached the monthly highest this year in March at over 30,000, but dropped to 22,400 in April and continued to fall last month.
In the first five months 113,500 units were sold, a 36% plunge year-on-year.
Thaco led in the period with 34,000 units sold, down 49%.
It was followed by Toyota with 21,500 units, down 43%.
Ford was the only brand in the top five with higher sales. It went up 87% to 14,300 units.
In the MPV priced under VND1 billion ($42,500) category, Mitsubishi Xpander led sales in the first five months with 6,700 units sold.
This was followed by the Toyota Veloz and Hyundai Stargazer.