Mix vinegar,2 tbsp. Worcestershire sauce,1 tbsp. oregano and the garlic. Pour over seasoned meat,slicing onion on top.Cover lightly; roast @ 350 degrees 3 hours or until done.Slice when cool. Before serving,combine sauce ingredients;bring to boil.Cool 10 minutes,then add sliced beef.Serve on poorboy buns.May be refrigerated 2 to 3 weeks.Makes 10 to 16 servings.