In a small bowl, whisk vinaigrette and garlic powder until combined. Stir in feta, cucumber, tomato and onion. Refrigerate, covered, until serving.
Brush eggplant with oil; sprinkle with salt and pepper. Grill, covered, over medium heat or broil 4 in. from heat 4-5 minutes on each side or until tender. Top eggplant with feta mixture. If desired, sprinkle with basil.