These days I see a few people riding around without helmets, mainly foreigners. They probably think it looks cool or cant be bothered putting it on. For a start it is illegal in Vietnam not to wear a helmet so ignoring this rule is a sign of disrespect to the country you are a guest in. Note you never see Vietnamese riding without a helmet now. I can also assure you a trip to the local hospital's emergency ward would soon convince you that you need to at least have some protection when a truck runs over your head and also by not wearing a helmet you will not covered by any insurance (even if you have it). so if you have a serious accident and need to be flown back to your home country you could be out of pocket for a lot of money. the helmet also protects your skin from the sun. so the moral of the story...don't be an idiot and wear your helmet !!