In Vietnam, preserving social harmony is key, and people will often keep quiet when faced with situations that would have the average westerner hopping mad. Face saving is probably one of the biggest hurdles foreign visitors must overcome to understand the Vietnamese, and foreign managers are renowned for being short tempered with their Vietnamese staff. Anger is often viewed as an indication of weakness, so getting angry in public is considered ill-mannered and uneducated. Above all, it's a loss face for the frustrated foreign manager, which leads to a loss of respect by his staff and anyone else who witnessed the outburst. Like concepts of good taste, ideas about what's polite vary a great deal from place to place.
Table manners are dangerous ground.
Especially in northern Vietnam, before starting a meal, younger people should invite their elders to begin. Never tap your bowl with your chopsticks and, when finished, place your chopsticks on the table, never pointing down in the bowl. This is said to resemble joss sticks an upsetting death image. If you happen to be hosting a Vietnamese family be sure to offer drinks and cake when they arrive. It's considered rude to simply ask if they'd like anything like it is in the west.