This is straight out of the guide book . . .

"Telling the Vietnamese that youa re single or divored and enjoy a life without children may disturb them greatly. Not having a family is regarded as bad luck and such people are to be pitied, not envied. Many Vietnamese will ask if you are married and have children, If you are young and single, simply say you are 'not yet married' and that will usually be accepted. If you are not so young (over 30) and unmarried, it may just be better to lie."

I can attest to the fact that people don't try to hide their look of shock and dismay when they hear my age and marital status though not many have asked.