Mix vinegar, water and pickle spice in saucepan and bring to a boil. Let stand away from heat 2 to 3 minutes. Place uncooked roast in a seal able glass dish. Pour sauce mix over roast and put onions around it. Let set covered in a cool place (refrigerator) for 6 to 7 days. Turn roast every other day. Take roast out of sauce. Separate pickle spice from sauce. Use sauce to cook roast at 375ºF. for 3 hours. Cover half way through with aluminum foil. For gravy use the sauce from roast and add water to make 3 cups. Combine butter and flour together and simmer until golden. Pour sauce mixture over flour mixture and slowly bring to a boil stirring constantly. Serve with potatoes, corn or red cabbage.