Preparation Cook beans in large saucepan of boiling salted water until crisp-tender, 4 minutes. Drain. Transfer to bowl of ice water. Drain; pat dry with paper towels. Cook potatoes in large pot of boiling salted water until just tender, about 12 minutes. Drain; transfer to large bowl. Sprinkle vermouth over hot potatoes; toss gently and let stand 5 minutes. Whisk vinegar, shallot, and mustard in small bowl. Gradually whisk in oil. Pour over potatoes and toss to coat. Cool completely. Mix in green beans and parsley. Season to taste with salt and pepper. (Can be made 1 day ahead. Cover and refrigerate.) Serve cold or at room temperature.
Preparation Cook beans in large saucepan of boiling salted water until crisp-tender, 4 minutes. Drain. Transfer to bowl of ice water. Drain; pat dry with paper towels. Cook potatoes in large pot of boiling salted water until just tender, about 12 minutes. Drain; transfer to large bowl. Sprinkle vermouth over hot potatoes; toss gently and let stand 5 minutes. Whisk vinegar, shallot, and mustard in small bowl. Gradually whisk in oil. Pour over potatoes and toss to coat. Cool completely. Mix in green beans and parsley. Season to taste with salt and pepper. (Can be made 1 day ahead. Cover and refrigerate.) Serve cold or at room temperature.