There is absolutely nothing wrong with wearing the waistcoat(vest) with your PC(assuming it is a low cut 3 buttoned one), many would say you were undressed without it. For me, I prefer the waistcoat rather than the belt and if it were me, I would forget the plaid under any and all circumstances, but just so you know, because you are wearing a waistcoat(if you so choose) the wearing of the plaid does not make your PC and black tie any more ,or less, formal than wearing a dress belt instead.