In a medium bowl, combine cabbage, vinegar, sugar and salt. Mix well. Cover and let sit for 1 hour.
Place the bread slices on a flat work surface. Spread 1 tablespoon of butter evenly over one side of each slice of bread. Flip the buttered bread slices over and spread 1 tablespoon of thousand island dressing over each.
Warm a skillet over medium heat for 3 to 5 minutes. Layer each sandwich as follows: Slice of bread, butter side down, dressing side up, 1/4 cup cheese, half of the corned beef, half of the cabbage, 1/4 cup cheese and slice of bread, butter side up.
Place sandwiches onto the hot skillet. Cook on each side for about 3 to 4 minutes, or until the bottom layer of cheese begins to melt around the edges. Cut in half and serve!