For extra flavor, tomatoes are marinated in a oregano vinaigrette before, and after, grilling.
Heat grill over medium heat. In a small bowl, whisk together vinegar, oil, oregano, salt, and pepper. Pour vinaigrette into a large shallow dish. Place tomatoes, cut side down, in dish. Let marinate 30 minutes to 1 hour.
Place tomatoes on grill, cut side down, and reserve marinade. Grill tomatoes until charred, about 3 minutes per side. Return grilled tomatoes to the vinaigrette. Let marinate again at room temperature until ready to serve.