Data from Vietnam Customs shows that Indonesia accounted for 53 percent of 28,523 imported cars with nine seats or less.
Thailand came second with 37 percent, losing the top place it has been enjoying since 2016.
The two countries account for 90 percent of Vietnam’s passenger car imports.
Cars imported from Indonesia include the multi-purpose vehicle Mitsubishi Xpander, among the top 10 best-selling cars last month, and other models such as the hatchbacks Toyota Wigo and Honda Brio.
January-May passenger car imports fell 40 percent year-on-year to 28,523 units as domestic demand dwindled and factories in Thailand and Indonesia were shut down to contain the novel coronavirus.
In the same period, imports of car parts fell 19.6 percent year-on-year to $1.3 billion.