Position a rack in the center of the oven and heat the oven to 425°F. Put the potatoes on the oven rack and bake for 30 minutes. Prick each potato on all sides with a fork. Continue to bake until the potatoes are tender when pierced with a fork, about 15 minutes more. Carefully remove the potatoes from the oven.
While the potatoes are still hot, hold each one with a clean kitchen towel and cut off about one-quarter lengthwise from the top. With a spoon, gently scoop the potato flesh into a large bowl, leaving enough flesh attached to the skin that it holds its shape. Scrape any flesh from the tops and discard the skin (or eat it as a snack).
With a fork, mash 4 Tbs. of the butter into the potatoes until melted. Add the sour cream, chives or scallions, 3/4 cup of the cheese, and the vinegar. Season to taste with salt and pepper. Don’t overmix—it’s fine if the mixture has a few lumps.
Spoon the filling into the reserved potato shells, mounding it nicely and making sure the filled shells are resting upright. (The potatoes may be prepared to this point up to 1 day ahead; wrap each in plastic and refrigerate.)
When ready to serve, arrange the potatoes on a baking sheet or in a baking dish. Cut the remaining 2 Tbs. of butter into small pieces and dot the potatoes with the butter. Sprinkle the potatoes with the remaining cheese. Bake at 425°F until heated through and starting to brown on the top, 20 to 25 minutes. (If baking straight from the refrigerator, allow an additional 10 to 15 minutes for the potatoes to heat through.)