Thread onions onto skewers (or put in grill basket), then brush with 1/2 tablespoon oil and season with salt and pepper. When fire is medium-hot (you can hold your hand 5 inches above rack for 3 to 4 seconds), grill onions, turning occasionally, until tender, 16 to 20 minutes. Transfer to a cutting board. When just cool enough to handle, cut onions into 1-inch pieces and toss with vinegar and 1/2 teaspoon salt.