Too crazy you found that!
I personally feel there is a slight differance between 5-20mph on a moped where EVERYONE is riding (vietnam) vs. 1 motorcycle per hundreds if not thousands of cars moving 30mph+ (US).
I wore a full face helmet helmet when I rode my R6 along with an armored jacket, gloves, and jeans and thankfully I never used them.
But lets not turn this into a "should I be forced to wear a helmet" thread (those belong in another forum). This is just an informative post in the SE Asia forum so that if anyone is on thier way over or has family over there they will know that it is now required to wear a helmet in Vietnam.