Heat grill to medium-high. Line a plate with paper towels.Place potatoes in a large saucepan and cover with cold salted water by 2 inches. Cover and bring to a boil, reduce heat and simmer until easily pierced with a knife, 14 to 16 minutes. Drain and let cool. Transfer to a bowl and toss with 2 tablespoons of the olive oil.

Meanwhile, cook bacon in a large skillet over medium heat, stirring occasionally, until crisp, 5 to 7 minutes. Remove to prepared plate with a slotted spoon.Discard all but 1 tablespoon of the bacon drippings from skillet. Off the heat, add garlic, vinegar, and brown sugar to the drippings in the skillet, scraping up any browned bits. Whisk in remaining 4 tablespoons oil. Season with salt and pepper. Set aside.

Grill potatoes cut-side down until lightly charred, 2 to 3 minutes. Grill scallions, turning occasionally, until charred, 4 to 6 minutes. Chop scallions into 1-inch pieces.Place potatoes and scallions in a bowl and toss with the reserved dressing to coat. Let stand for 5 minutes. Sprinkle with bacon, and toss to coat. Taste and adjust for seasoning with salt and pepper. Garnish with herbs. Serve immediately.