Cut ½ inch off the top of each apple. Using a grapefruit spoon or sharp paring knife, remove the core and seeds, leaving the bottom intact. Prick the apple skin in a few places. Place the apples in an 8-inch-square baking dish. Place 1 teaspoon butter pieces in each apple. Stir syrup, cinnamon and vanilla together in a small bowl. Divide among the apples. Pour the water into the bottom of the baking dish, cover with foil and bake until the apples are soft but not mushy, about 40 minutes.