In a large pot of boiling water, cook potatoes until they are tender. In a large bowl whisk together the vinegar and the mustard. Season with salt & pepper. Whisk in the olive oil a third at a time until it binds together. Add the warm, drained potatoes and toss to coat. Let the potatoes sit for 10 minutes or longer to absorb the flavor of the dressing. Add the scallions and parsley and toss to coat. Season well with salt & pepper.