I've always been taught that wing-on-wing is really only for VERY light wing days when it's difficult to get the sails to fill. Just so we're on the same page, wing-on-wing is when you main sail and fore sail are on different sides of the boat. Typically someone has to hold the foresail out in order to get it to fill on a light day (hence wing-on-wing).
A preventer is a device (from just some simple rope to more complicated versions) that will actually prevent the boom from gybing).
More info here: Preventer - Wikipedia, the free encyclopedia
The boom vang (here: Boom vang111 - Wikipedia, the free encyclopedia
is more of an advanced sail control that allows you to change the shape of the sail by exerting downward force on the boom.