A target is something to be aimed at, whether with a bow, gun, catapult, or other object; or with humour, as in a mission or MST. Targets may be objects or persons, depending on context.
A vulnerable target is one that is especially easy to aim at or pick on, depending on which form of target one is referring to.
An acceptable target is one you're allowed to aim at or pick on. In the PPC, Mary Sues are considered acceptable targets for both weapons and mockery, and bad fanfics in general are an acceptable targets for all forms of sporking. However, their authors and other real people are not.