Cook bacon in a 12-inch skillet over medium heat until crisp, 7 to 9 minutes. Transfer to a paper towel-lined plate. Set aside 2 tablespoons of bacon fat. When cool enough to handle, crumble the bacon and set aside.

Whisk together the vinegar, mayonnaise, chile, ½ teaspoon salt and ½ teaspoon pepper in a large bowl for the dressing. Slowly whisk in 3 tablespoons of oil until combined. Set aside.

Charcoal Grill. Open the bottom vent completely. Light a large chimney starter three-quarters filled with charcoal briquettes (4½ quarts). When the top coals are partially covered with ash, pour evenly over the grill. Set the cooking grate in place, cover, and open the lid vent completely. Heat the grill until hot, about 5 minutes.

Gas Grill. Turn all the burners to high, cover, and heat grill until hot, about 15 minutes. Turn all burners to medium.

Clean and oil the grill grates. Toss the potatoes with the reserved bacon fat and ½ teaspoon salt. Push a toothpick horizontally through each onion round to keep the rings intact while grilling. Brush the onion rounds lightly with extra oil and season with salt and pepper.

Place the potatoes (cut side down) and the onion rounds on the grill and cook, covered, until charred on first side, 10 to 14 minutes.

Flip the potatoes and the onion rounds and continue to cook, covered, until well browned all over and potatoes are easily pierced with the tip of a paring knife, 10 to 16 minutes longer. Transfer the potatoes and the onion rounds to a rimmed baking sheet.

When cool enough to handle, halve the potatoes. Remove the toothpicks and coarsely chop the onion rounds. Add potatoes, onion, scallions, and bacon to the dressing and toss to combine. Season with salt and pepper to taste. Serve warm or at room temperature.