Is safe to ride a bike during the typhoon
The simple answer to this is no. Many roads in Vietnam are unsealed and become very slippery in the rain. Depending on where you are, busses may be cancelled until the roads have dried out. Do not, under any circumstances, drive a motorbike in a typhoon regardless of how desperate you are to get to your next destination. Rain plays havoc with both sealed and unsealed roads and the ratio of potholes to road only gets worse. This makes the roads deadly and it is not worth risking your life.
The soft fertile soil that makes up most of Vietnam is prone to slips and landslides after heavy rain so avoid roads that have steep drops on one side for a few days after the typhoon has passed.