Heat a large cast iron skillet over high heat. Spray with nonstick cooking spray. Add the sliced tomatoes without crowding. Cook on one side until the undersides are blistered, about 3 minutes. Flip and cook an additional 2 to 3 minutes until the tomatoes are tender and speckled on both sides. Set aside and keep warm.
Add the corn to one side of the skillet and the onions to the other. Cook until the vegetables are blistered and charred, 3 to 5 minutes. Transfer the corn and onions to a bowl. Season with salt and pepper. Add vinegar, oil, cherry tomatoes, and herbs. Toss to combine.
Transfer the corn and onions to a platter. Top with griddled tomatoes. Serve immediately with dipping sauce on the side.