Vietnam has plenty of places where you can ride a bicycle, regardless of terrace. The ride is extremely enjoyable if you are on the beach or on a scenic route like Dalat.
There are many organized bike tours for mountains or down the countryside area. Avoid riding the bicycles in cities where you have to breathe in the pollution and experience annoyance because of fellow motor bike drivers.
Most travel agents or hotel receptionists will kindly assist you with renting a bicycle. The price varies, depending on cities and make sure you are not charged more than $5 per day of rental.