1. Whisk together vinegar, mustard, salt, and pepper. Add oil in a slow stream, whisking until emulsified. Put oven racks in upper and lower thirds of oven and preheat oven to 425°F.
2. Wrap beets individually in foil and roast on a baking sheet in upper third of oven until tender, 1 to 1 1/4 hours. Once beets have roasted for 30 minutes, toss potatoes with oil and salt in a small baking pan and roast in lower third of oven, shaking pan occasionally, until potatoes are tender, 25 to 30 minutes.
3. Carefully unwrap beets and cool slightly, then slip off and discard skins. Cut beets into 1/3-inch dice and put in a large salad bowl. Cut potatoes into 1/3-inch-thick slices and add to beets along with all greens and herbs. Add vinaigrette and toss gently to coat.