Waistcoats may be > to cummerbunds but they are not interchangeable with the same outfit. A Jacket of a dinnersuit should be matched with its trousers and waistcoat, the cut is slightly different (three piece jacket buttons higher than its same style two piece equivalent) and the material of the jacket (lapels and the rest) should match the materials used for the trousers and waistcoat (if three piece), as it is part of a set. Due to the jackets cut a three piece suit used as a two piece with a cummerbund will look a little off, the same way a two piece suit with a waistcoat looks odd even if the materials are the same. Cummerbunds should match the bow tie and obviously shouldn't be used in combination with a standard tie.
So unless you have a two piece and a three piece to choose from or you are looking to buy a new suit, your decision is made up for you. Wear what you have or buy a new suit, don't mix and match two parts of a three piece suit with a cummerbund or a two piece suit with a waistcoat. Most people wont pick up on it and if it is an event where dressing up is just part of the fun, it doesn't matter but if the event requires dressing up, i would avoid any fashion faux pas for the sake of fitting in.