In a small bowl stir together yogurt, tomato, oregano, and cumin. Cover and refrigerate until ready to serve.
Stack tortillas and wrap in heavy foil; set aside. Brush vegetables lightly with oil; sprinkle with chili powder.
For a charcoal grill, place onion slices on the rack of an uncovered grill directly over medium coals. Grill for 5 minutes. Turn onion slices. Add remaining vegetables and tortillas. Grill for 6 to 8 minutes more or until vegetables are tender and tortillas are heated through, turning vegetables occasionally. (For a gas grill, preheat grill. Reduce heat to medium. Cover and grill onion slices, as above. Add remaining vegetables and tortillas on grill rack directly over heat. Cover and grill as above.)
To assemble, place the grilled vegetables on warm tortillas. Top with some of the yogurt mixture; roll up tortillas. Pass the remaining yogurt mixture. Makes 2 servings.