One thing I found out in my travels to Vietnam, is that you should not tip anyone. The reason why, is because Vietnamese are not interested in taking charity, as they feel it makes us lose face and feel like we're begging.
I never thought of it that way. I did tip my first time eating in Vietnam, and the server didn't accept it. My friend who came with me, said not to tip, as it was frowned upon. So I never tipped again.
Have you ever made the mistake of tipping?