For some reason, most Vietnamese people ride under-175cc motorbikes or scooters. Firstly, the bigger motorbikes have to be imported, and the government charges an import tax of at least 100%, along with a huge fee for registration, so generally, it may cost twice as much as in a foreign country (it’s too expensive for most people in Vietnam). Secondly, Vietnamese traffic law allows a motorbike’s maximum speed of 40 km/h in the city and 60 or 70 km/h on highways. So the bigger bikes seem to be unnecessary in Vietnam.