Vouchers are available from several different internet vendors and what you would do is purchase them at a set price for adults and children, then use them at a participating location as a fixed price meal that includes generally an appetizer, entree, dessert, and sometimes a snack. I've never used them though but there is a lot of information about them on the budget board.
Wishes are an add-on that you can get with a package plan. There are a few grades of wishes (silver plan and gold plan for examples) and they are also fixed price per day. I think when you add them on to your package you have to get them everyday and not just for part of your vacation (under the same reservation). These can be used for different dining options and special events/shows around WDW. Depending on the dining experience or special event you may have to use 1-2 wishes per adult. The trick with wishes is to make sure that you get your money's worth in how you "spend" them. Again, I have never used them so I don't know the price.