Prepare grill for medium-high heat. Combine chile and 2 tablespoons vinegar in a small bowl; set aside. Place onions on a rimmed baking sheet and drizzle with 2 tablespoons oil; season with salt and pepper. Turn to coat. Grill onions directly on grate until lightly charred and softened, about 2 minutes per side. Transfer to a large bowl and toss with remaining 2 tablespoons vinegar; let cool. Coarsely chop 1/2 cup grilled onion and return to bowl.