Sales for the top five brands, which account for around 95 percent of the market, had fallen 6.13 percent in the first quarter and 4.39 percent in the second, according to the Vietnam Association of Motorcycle Manufacturers (VAMM). 

Honda has a nearly 77 percent market share, with Piaggio, Suzuki, SYM, and Yamaha making up the top five.

In the first nine months together they sold around 2.33 million units, down 5 percent, VAMM data shows.

Although motorcycles remain the major mode of transport, the increasing frequency of traffic jams and air pollution, especially in big cities, are causing their sales to gradually fall, local experts said.

VAMM had earlier said that falling motorbike sales were because the market was near saturation. It has decided not to hold its annual motorcycle exhibition this year for this reason.

With sales of nearly 3.4 million units last year, Vietnam was the world's fourth biggest motorcycle market after India, China and Indonesia, according to a recent report by market research firm Motorcycles Data.

Last year, Vietnam had the highest proportion of people buying new motorbikes, with over 35,000 new motorbikes sold per one million people.

At the end of 2016, there were 45 million registered motorbikes in Vietnam, a country of over 92 million people, according to the Ministry of Transport.