Never touch anyone on the head, which is sacred. Go a little lower an touch their shoulder instead.
Avoid shouting in public. To attract someone’s attention, clap.
To beckon, turn the palm down and wave up and down. A palm up signal is impolite.
Always were a shirt when visit come.
Wear long trousers or skirt and cover the shoulders when visiting temples or churches. Some touristy temples supply sarongs.
Wear polite clothes (see above) at government offices. If not (1) expect to be sent home to change, or (2) told to sit down and wait. They will let you leave when the office is closing for the day.
Many Immigration offices simply do not want to see you and will waste your time if you go there. It is usually easier to go to have a travel agent sort out your visa for you.
As others have written, try and learn some Vietnamese. People will appreciate that you have tried, and the more you know the more doors of friendship will open.
Smile. Young woman will always smile back and this will make your day.
When visiting an office or a Vietnamese family, wear sandals or shoes to be polite. Do not wear thongs/flipflops.
Even in the street, wear clean and neat clothes; bathe before leaving your hotel, comb your hair. Vietnamese do not appreciate Westerners who dress like they have just crawled out of a rubbish tip. Russians walking down the street in their H-front underpants is just the worst example I have seen. As a landlord once asked me, “Why do they do this? They are supposed to come from a civilised country?”
Before eating or drinking, invite others first by saying, “Mời ăn cơm” - Please eat, or “Mời uống nước” - Please drink. Even strangers in the next table in restaurants do this. The polite answer is to smile and say, “Mời” in return.
When visiting invite others to eat and eait until they start eating first. In some northern houdeholds, the inviting goes on forever, especially at formal dinners. Be patient.
The hostess will appreciate your bringing flowers and the host will enjoy your gift of a bottle of brandy, the preferred drink of most Vietnamese men.
Generally speaking, Vietanamese are completely turned off by facial hair. Young men, take note. And clean your teeth.