Toss the shallots with the vinegar in a small bowl. Let marinate at room temperature for 30 minutes.
Preheat the oven to 400°F. Place a wire rack over a large rimmed baking sheet and arrange the bacon slices on the rack. Bake for 15 to 18 minutes, until the bacon begins to brown. Remove from the oven and brush the maple syrup over the bacon. Bake for 3 to 5 minutes, until the bacon is golden brown and crispy. Transfer to paper towels to drain.
Place half of the bread slices on a work surface. Spread ½ tablespoon of mayonnaise over each slice. Top each with ¼ cup of arugula, 1 slice of tomato, and 3 slices of bacon. Lift the shallots from the vinegar and distribute among the sandwiches. Place the remaining bread slices on top. Cut each sandwich in half and serve.