"Bulletproof" vests were still sutler items as late as the end of 1862.

In December of 1862 - after his first exposure to battle - (Prairie Grove Arkansas) C. W. Dewolf of the 7th Missouri Cav wrote on December 11th. "I went up to the sutler's today and invested in a cot, cap, and bulletproof vest and feel a little doubtful about the utility of the last investment, the thing being too heavy to ride with."

Seems a very reasonable reaction to seeing the aftermath of a battle for the first time and not wanting to join the list of wounded or dead.