What part of Vietnam you are going to and the time of year will determine what clothing to bring. The Northern part of Vietnam has 4 seasons, whereas the South is either in the dry or rainy season. Temperatures in the North can get close to freezing in winter. The South is always warm / hot and humid. Although you can buy clothing in Vietnam, if you are over 5 feet 9 in (1.75) you might well have problems getting clothes to fit without going to upmarket stores (with prices to match). There is no dress code except when going to temples etc. It will usually be very hot and humid further south, so hats and lots of water are the order of the day.
The Vietnamese love westerners, be prepared for lots of questions. Children will be adored, especially if of fair complexion as it is so different. The roads are chaos: countless thousands of motorbikes carrying families, fridges, pigs- its madness. Also, crossing the road takes extra care as bikes do not stop; they go around you. Not for the faint-hearted. Always look both ways when crossing as motorbikes will quite frequently run along the roads the wrong way. They are also driven and parked on the pavements. While the pavements are fine in some places, in a lot of others you will end up walking in the road. This is normal.