First, make the marinade for the lamb. In a bowl, combine the vinegar, dried chilies, cinnamon stick, cloves, and cardamom. Allow to soak for 20 minutes to soften the chilies. Pour into a blender and add the cumin seeds, mustard seeds, black peppercorns, garlic, ginger, coconut milk, and salt. Puree for 2-3 minutes until completely smooth, adding extra vinegar to get a paste that's not too thick. Pour the marinade over the lamb, rub, and marinate in the refrigerator, covered, for up to 4 hours.