I tried to do a web search on the subject before starting the topic, but since I have "safe search" turned off, I will not mention the sort of things that came up.
However, just about any tourist, male or female, who has visited Vietnam has had this happen, often more than once. It seems to be an "I-dare-you-to-do-it" game in the age group from about 10 to 15, and the kids seem to think that it is really great fun.
Sometimes, even an adult with surprise you with a furtive pinch, and once I even had another very personal body part squeezed to great mirth.
I don't have the slightest idea if this is reserved exclusively for tourists, or if the Vietnamese also do it among themselves.
It was amusing taking the tourist buses in Vietnam, because you always talk to total strangers at the rest stops. People of both sexes would start their anecdote with "We were at such-and-such temple the other day. There were all of these kids hanging around and then the strangest thing happened!"
If you guess "one of the kids ran up and pinched a nipple," you are right about 80% of the time.
When it happens the second time, you just brace yourself for it. What if you slap a kid and he doesn't pinch your nipple? What if the police are 10 meters away?
SE Asians have a very different approach to showing different emotions and these are often misread by visitors, ie. two males holding hands (usually little fingers) while walking only signifies they are friends and heading in the same direction, nothing more. I believe your nipple assault is just an attempt to draw a reaction from you be it shock, surprise, anger, ect. During the Vietnam conflict many a GI found themselves groped and usually the topic of conversation among locals.