Vietnamese people have a habit of not looking into your eyes when they talk to you. This is often because of shyness, but one of the main reasons is that traditionally they do not look into the eyes of those they respect or those higher in rank when talking to them. This is to indicate politeness.
The smile of a Vietnamese can be very confusing in Vietnam to an outsider and cause misunderstandings. In some Oriental countries, a smile can mean sorrow, worry, or embarrassment. In Vietnam, it may indicate a polite, but perhaps skeptical reaction to something, compliance or toleration of a blunder or misunderstanding, or on occasion represents submission to judgment that may be wrong or unfair. This is particularly true if the one making the judgment is at a superior level and perhaps has lost his temper. For instance, a laundress may ruin a favorite shirt and is called in by her employer to be asked about it. She may smile. This does not mean that she thinks it is funny that she burned the shirt, but instead is submission to the fact. If the owner of the shirt loses his temper, she may keep smiling indicating politeness or patience with superiors.
Because of this, foreigners should be very cautious in voicing their opinions and perhaps be a little more delicate, more tolerant and restrain from being obstinate.
Loud arguments or heated discussions are frowned upon and are seldom heard among the Vietnamese. Well-bred people are trained in self-discipline. It is best, therefore, for Americans or other foreigners to do their best to keep tempers in check, no matter what the circumstances, lest they be looked upon with disdain.
Vietnamese seldom use a direct approach in their dealings. To do so indicates a lack of tact or delicacy. Directness is appreciated in the Western world, but not in Vietnam. The Vietnamese do not like to say "no" and will often reply "yes" when the answer should be negative. This problem is further complicated by Americans posing negative questions such as, "It doesn’t look like it will rain today, does it?" The correct answer is often the one given by the Vietnamese--"Yes." We expect to hear "No." Think it out and you will see that the Vietnamese is really correct.
Best advice, don’t ask negative questions.