Place the vinegar and rosemary in a blender and blend until smooth. Slowly add the olive oil until emulsified and season with salt and pepper to taste. Strain into a bowl. Preheat grill. Toss potatoes in a large bowl with olive oil and season with salt and pepper to taste. Grill skin side down until golden, turn over and continue grilling until just cooked through. Place in a large bowl and immediately toss with 1/4 cup of the vinaigrette. Meanwhile, bring a large pot of salted water to a boil and add the haricots verts. Cook for 2 minutes, and immediately plunge into a bowl of ice water. Drain. Add the haricots verts and red onion to the potatoes and drizzle with the remaining vinaigrette. Season with salt and pepper to taste.