I have never heard of this Challenge coin until today. My Father was Career Old Corps I am Vietman Corps and never heard of the coin from my Father or any of my fellow Marines. This may be an Army tradition or to new for me to have known about it.
I think Challenge coins have been around since Gulf War
you are right, were not around during Nam
http://www.3ad.org/coins/coins.htm
Many units in the U.S. Army have coins. These coins probably came into common use sometime in the 1970’s or 1980’s, and were generally given out by the unit commander as a sort of “on the spot” award for performance or as gifts to visitors to the unit. In addition to the more standard “Commander’s Coin”, there were sometimes Command Sergeant Major’s coins. Another subset were coins commemorating specific events such as Operation Desert Storm.
Some more information on the “coins”
http://www.dod.mil/dodgc/defense_ethics/resource_library/2005Deskbook/TabD/Military_Coins.doc