Mashed Potato Salad
1-1/2 pounds red potatoes (about 5 small potatoes), scrubbed but unpeeled, cut into chunks 1-1/2 pounds white potatoes (about 5 small potatoes), peeled, cut into chunks 1 tsp white vinegar 3/4 cup mayonnaise 2 Tablespoons prepared mustard 1/4 cup sour cream 1 rib celery, diced…