Dress
The Vietnamese often dress in more traditional clothing, but in cities and among the younger generations this is quickly changing. The traditional outfit for women is called an ao dai, which is a loose-fitting outfit similar to a dress and is often brightly colored. Men tend to wear what is called an ao the, which is a loose-fitting shirt or tunic. However, men have turned to Western clothing in greater numbers than women in Vietnam so western clothing and suits are also common. With the traditional clothing, large-brimmed sun hats are common, especially for those working in the muddy fields. With this clothing long pants are the norm; but in some places, like muddy fields, the pants are pulled up to appear as shorts in order to prevent them from getting muddy, however most pants and shirts are long sleeved.
Despite the traditional dress, both men and women are turning to western clothing in greater numbers. As mentioned, men are more commonly seen in western clothing, but women seem to still dress in traditional clothing in fairly large numbers.
There are few dress restrictions in Vietnam and there is no expectation that you dress like the locals. Although few locals wear shorts, there is no discourtesy involved in wearing them as this is a cultural phenomenon, not a religious rule or cultural offense. Of course if you are visiting temples or other sights of particular religious, historical, or political importance you may be expected to cover up with long pants and long-sleeve shirts.