In large bowl, whisk vinegar, oil, salt, and black pepper. Add cantaloupe and cucumber and toss until coated. Let cantaloupe and cucumber sit for 5 to 20 minutes.
Meanwhile, coat grill pan with nonstick spray. Heat pan over medium-high. Coat cheese with nonstick spray. Grill until grill makes appear and cheese is softened, about 2 minutes on each side.
Arrange salad on platter or in shallow bowl. Top salad with warm grilled cheese and serve immediately.
Serving size: about 1 cup salted and 1 cheese slice.