In large saucepan over medium heat, combine cider vinegar, water, brown sugar, cloves, allspice, salt, pepper, peppercorns, bay leaf, onion, carrot and celery. Heat, stirring occasionally, until bubbles appear at edges, but do not boil.
Poke deep holes in roast and place in non-metal bowl. Pour vinegar mixture over roast. Cover and refrigerate for five days, turning once daily.
Remove roast from liquid; pat dry with paper towels. Remove bay leaf and peppercorns from liquid and discard. Strain liquid from vegetables, reserving both.
Heat oil in Dutch oven over medium-high heat. Sear all sides of roast quickly, 1 to 2 minutes each side. Reduce heat to low, pour reserved vegetables and 1 cup of vinegar liquid into pan. Cover and simmer 1 1/2 hours, adding more vinegar liquid if necessary.
Remove roast from pan, and let cool slightly. Slice roast and return slices to liquid in pan, cover and simmer 30 minutes more. Remove slices to serving platter. Stir sour cream into heated liquid and heat through, but do not allow to boil. Pour sauce over meat and serve immediately.