In a very large nonstick skillet, heat 2 tablespoons of the olive oil until shimmering. Add the potatoes in a slightly overlapping layer and cook over moderately high heat, stirring occasionally, until golden and nearly cooked through, about 8 minutes. Add the onion, season with salt and pepper and cook, stirring occasionally, until tender, about 5 minutes longer. Add 1/2 tablespoon of the vinegar and gently toss to combine. Cover loosely and keep warm.