Combine first 6 ingredients in a small bowl, stirring well with a wire whisk. Place sweet red pepper and next 4 ingredients on a baking sheet; brush vegetables with half of vinegar mixture.
Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place vegetables, cut side down, on rack; grill, covered, 5 minutes. Brush with remaining vinegar mixture; turn vegetables, and grill an additional 5 to 6 minutes or until vegetables are tender. Remove vege-tables from grill; cut onion slices into quarters. Garnish with oregano, if desired.