In large bowl whisk together pesto, vinegar and olive oil.
Divide chicken into two gallon-size freezer bags (be sure to label bags first!) Divide sauce into bags over top of chicken. Freeze until needed.
Thaw; Grill 5-7 minutes on one side, then turn over and grill 5-8 minutes longer. With a minute or two to go, place sliced tomato over chicken, then sprinkle with mozzarella. Close lid and let cheese melt completely.
OR: Arrange chicken in casserole dish; place tomato slices over chicken, then sprinkle heavily with mozzarella cheese and bake at 400 degrees for 30-40 minutes. Serve over cooked pasta.
Chicken can be cooked 10-15 minutes on the grill or 30-40 minutes in the oven.