Bicyclists usually don't have headlights. If you get into an accident, assume it's your fault. Pay off the other driver quickly and get away before a crowd gathers and the police get involved. If you park in established parking lots you'll be given a receipt for your bicycle. Don't lose it.
Buses stop frequently and often without warning.
There are several makes and manufacturers of bicycles in Vietnam. If you plan to pedal in the Tonkinese Alps bring a many-geared bike and calves of steel. Also be sure to bring whatever special tools you need and all necessary replacement parts. I believe that their gear systems are made out of butter and their spokes out of uncooked noodles. Buying one of these is a great way to get to know the local mechanics.