What to wear in Vietnam
The main thing to consider is the weather. It can be freezing cold in the Northern mountains, and at the same time incredibly hot and humid on the Southern coast. If you are traveling in the North or the Central Highlands during the winter months definitely bring long pants and a warm coat or sweater (preferably not cotton). It seems that it is always raining somewhere in Vietnam, so lightweight, breathable rain gear is essential.
In the hot summer months, dress cool but conservative. Many Vietnamese cannot understand why foreigners insist on wearing short-shorts, tank tops and sleeveless t-shirts when they have the money to dress well. For the Vietnamese, appearance is very important, so if you are dealing with or as an official of any rank, make sure you are dressed appropriately.
The rule on shoes are a little more lax. Flip flops are a must on your travels. It is okay to wear leather sandals to a business meeting. Proper, broken-in footware is needed if you plan on doing any trekking.
A summer hat and sunglasses are also advised.