Present at a conversation between two Vietnamese colleagues, you notice that no “thank you” was said even though one colleague gave the other a very expensive gift. You, should take this to mean:
A. The colleagues are embarrassed because they do not have a gift for you.
B. The receiver takes it as a casual act between friends, hence no politeness is needed.
C. The Vietnamese do not always say “thank you.”

Phép vua thua lệ làng.
-Vietnamese saying-

Risk -: Your family adopts a homeless cat. You tell this to your Vietnamese colleagues and they praise you for your generosity. One day, on the way from the vet, you drop by your office to show them the cat, totally unaware that Vietnamese may welcome a strange dog but avoid letting in a strange cat. This habit is probably just superstition, as a dog’s bark resembles a Vietnamese word meaning "rich", while a cat’s meow sounds like a word meaning "poor".