TOSS spinach with tomatoes in large serving bowl; set aside.
COOK and stir bacon and onions in large skillet on medium heat until bacon is crisp; discard drippings in skillet. Return bacon mixture to skillet. Add vinegar, water, sugar and pepper; mix well. Cook 3 min. or until heated through, stirring frequently.
ADD to spinach mixture; toss to coat. Serve immediately.