I've not read the Vertigo series but in the original The Losers war comics, here's why they dubbed them selves that via Wiki
Upon America's entry into the European theater of the Second World War, a Special Forces group composed of personnel from all branches of the armed forces was formed. The members were Captain William Storm, a former PT Boat commander with a wooden leg and an eye-patch; Sarge Clay, one of the oldest serving Marines; Gunner Mackey, one of the youngest; and Johnny Cloud of the U.S. Army Air Force, known as "The Navajo Ace". Because each member had suffered the deaths of personnel under their command for which they felt responsible, they dubbed themselves The Losers.