Steam potatoes until just tender, about 12 minutes. Transfer to large bowl. Cool potatoes slightly. Drizzle with vinegar; toss to coat.
Heat oil in heavy medium skillet over medium-low heat. Add onion; sauté until beginning to soften, about 2 minutes. Stir in garlic and Old Bay seasoning; sauté 1 minute. Remove from heat. Cool to room temperature. Add onion mixture to potatoes. Add remaining ingredients. Toss gently to combine. Season salad with salt and pepper. (Can be prepared 1 day ahead. Cover and refrigerate. Bring to room temperature before serving.)