In a small bowl, whisk the vinegar with the mustard. Whisk in the olive oil and season with salt and pepper. Add half of the dressing to the potatoes along with the bacon and half of the tomatoes; season with salt and pepper. In another bowl, toss the lettuce, scallions and remaining tomatoes with the remaining dressing and season with salt and pepper. Transfer the greens to 4 plates, top with the potato salad and serve.