Enlisted Marine Corps vests.

[major bill]

In the years leading up to the Civil War Marine Corps officers were authorized vests. By the time of the Civil War civilian use of vests had become fashionable. Because Marine Corps uniform regulations did not forbid vests for enlisted Marines, they soon started to wear vests. The lack of regulations resulted in there being no standard enlisted Marine Corps vest. Dark blue vests appear to be worn as well as light blue vests that matched the trousers. Small Marine Corps buttons were often the button of choice. The vests not only helped keep the men warm but were considered fashionable.

 

[mofederal]

I think vests were just a popular item, they did tend to look smart, and the warmth part is true especially on a hot day in the sun. They also helped in colder weather also, but they completed a well rounded and fashionable look. Taking off your coat you needed a vest to be properly dressed.