Heat grill. In blender container or small bowl, combine vinegar, thyme, salt, oil and garlic; blend or stir until well mixed.
When ready to grill, carefully oil grill rack. Brush both sides of eggplant and onion slices with vinegar mixture. Place eggplant and onions on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium coals; cover grill. Cook 5 minutes.
Brush tomato halves with vinegar mixture. Place tomatoes, cut side down, on grill. Turn eggplant and onions; brush with vinegar mixture. Cook 5 minutes.
Turn all vegetables; brush with vinegar mixture. Cook an additional 5 to 10 minutes or until onions are crisp-tender.
To serve, brush remaining vinegar mixture over vegetables; sprinkle with pepper. Place on serving platter.