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. And since since Vietnamese women seldom drink beer or coffee, have some green tea or soft drinks or hand.
Clothing is another potential trouble spot.
When on holiday, the Vietnamese tend to dress up and are often aghast at the disheveled state of many backpackers. Giving gifts in Vietnam can be an unfulfilling affair for a Westerner. Often gift the recipient simply acknowledges the gift and carries on as before. Presents are rarely unwrapped in the giver's presence and many foreign visitors feel put out by the lack of visible gratitude. There is simply no tradition of ingratiating yourself when you receive a present in vietnam culture.