This is related to the fact that knitted thin garments with full-length zippers [usually designed for sports activities, golf, rowing, etc] were called vests in the USA [if I remember correctly during the 30s]. That's why C-2 was named vest in mil-spec. Cardigans have in general a visible v-neck and zipped woolen vests didn't have this feature. That's why calling a knitted true-vintage garment with a whole-length zipper a vest is correct [ was correct a long time ago]For some odd reason, the specs of the C-2 referred to it as a vest. I bought a Willis & Geiger repro C-2 many years ago and remember them referring to it as a "vest." Can't answer why.