For the vegetables: Prepare a grill for medium heat cooking; bring the grill to 400 degrees if your grill has a thermostat. Trim and slice the zucchini and summer squash lengthwise. Cut the tops off the bell peppers, cut them in half and pull the seeds out. Place the vegetables on the grill for 20 minutes.
After 20 minutes, turn all of the vegetable slices over, and grill for another 15 minutes. Use tongs to pull them off of the grill and place them on a platter.
Meanwhile, make the dressing: While the vegetables are on the grill, in a large jar with a tight-fitting lid, combine the olive oil, vinegar, parsley, cilantro, garlic, cumin, paprika, coriander, salt, and black pepper. Cover the jar and shake to combine. Set aside.
Drizzle the vegetables generously with the dressing and serve immediately.