Preheat the oven to 450 degrees F. Place a baking sheet in the oven to heat. In a deep skillet, bring the vinegar and sugar to a boil over medium-high heat. Stir until the sugar has completely dissolved. Add the coleslaw to the skillet. Stir well to coat in the sugar mixture. Cover the skillet and cook, stirring occasionally, for 6-8 minutes or until the cabbage is tender. Remove the lid from the skillet and let the mixture simmer until the liquid has cooked off. Sprinkle the cabbage with salt and pepper. Mix together the mayonnaise and mustard in a small bowl until smooth. Spread the mixture on one side of each bread slice. Layer 1 slice cheese, 1/2 cup sauteed cabbage, 1/4th of the corned beef, and another slice of cheese on the buttered side of one slice of bread. Top with another slice of bread, buttered side down. Press on the sandwich gently to compact the ingredients. Repeat with remaining sandwiches. Brush the top and bottom of the sandwiches with the melted butter. Place the sandwiches on the preheated baking sheet. Bake at 450 degrees F for 10 minutes or until the sandwiches are toasted. Serve hot.