Remove and discard mushroom stems. Place mushrooms, eggplant, and corn in a large roasting pan coated with cooking spray. Coat eggplant mixture with cooking spray; toss, and coat again with cooking spray. Bake at 450° for 20 minutes or until lightly browned, stirring occasionally. Let cool.
Combine eggplant mixture, cheese, and next 3 ingredients in a large bowl; toss lightly.
Combine vinegar and next 3 ingredients in a jar; cover tightly, and shake vigorously. Pour vinegar mixture over vegetable mixture; toss lightly.
Cut each pita round into 8 wedges; place on a baking sheet coated with cooking spray. Bake at 400° for 5 minutes or until crisp.
To serve, place lettuce on 5 plates. Top each with 1 1/2 cups vegetable mixture. Serve with toasted pita wedges.