It was only behind Indonesia where sales declined by 5% to 3.2 million units, according to statistics platform MotorCycles Data.
It forecast the Vietnam market could reach nearly three million units this year.
But the platform considers Vietnam’s sales figures below expectations.
"While the domestic economy is growing very fast, sustained by strong export, the market is constrained by low demand from the middle class and has yet to recover from Covid-19 shock."
Supply chain troubles and increasing raw material costs have pushed up prices for new models, which cooled down demand, it added.
The rest of the top five ASEAN markets comprise the Philippines, Thailand and Malaysia.
ASEAN saw 8.8 million motorcycles sold in the first nine months, up 6.4% year-on-year.