Position a rack in the upper third of the oven and preheat to 400 degrees . Line a baking sheet with foil. Place the bread on the baking sheet and lightly toast one side in the oven, about 3 minutes. Set aside.
In the microwave or a skillet, cook the bacon until crisp. Drain on paper towels.
In a medium bowl, stir together the mayonnaise and horseradish. Spread about a tablespoon of the mixture on each toast slice. Toss the coleslaw mix and vinegar with the remaining mayo mixture in the bowl. Season with salt and pepper.
Arrange 2 slices bacon on each toast slice. Layer the roast beef over the bacon and top each with a mound of slaw. Cover each toast with Swiss cheese and bake until the cheese is bubbly, about 5 minutes.