This situation is not super common but can happen especially to ladies that use a purse. When you walk on the streets make sure you use your purse in a way that is not easy for someone to remove it from you. There are some thieves that use motorbike and pull your purse if you don't pay attention. Avoid using large gold necklaces too because they can be an easy target. Most of the times Vietnam is really safe but if you can avoid these situations is better for you.