Preheat oven to 200C. Carefully score skin of pork in a 2.5cm diamond pattern just deep enough to penetrate fat without exposing flesh. Pound fennel seeds, garlic and 2 heaped tsp salt to a paste with a mortar and pestle. Rub all over pork, pushing into scored skin, then place skin-side up in a large deep roasting pan. Place fennel bulbs neatly around pork and pour in wine. Reduce oven to 100C and roast pork overnight (or at least 8 hours). Remove shoulder and fennel from pan and set aside covered to rest (20 minutes).
Meanwhile, add vinegar to pan and reduce cooking liquid over medium-high heat until sauce consistency (25-30 minutes). Keep warm.