Soak the ancho chile peppers in water to cover for 12 hours. Drain them, pat them dry, seed them, and set them aside.
In a saucepan, combine the vinegar, chicken stock, lemongrass, and ginger. Bring to a boil, then lower the heat and simmer until it is reduced by one half, approximately 20 minutes. Strain the mixture and reserve, discarding the solids. Return the liquid to the saucepan.
Add the garlic and seeded anchos to the reserved liquid, simmer for 15 minutes, and purée in a food processor or blender.
Add the cilantro, lime juice, and coconut milk, and mix well. Season the mixture with salt and pepper to taste. Keeps up to 3 days, covered, in the refrigerator.
When you are ready to grill, place the pork pieces between two pieces of plastic wrap, and lb them until evenly thin but not torn.
Season the pork pieces with salt and pepper to taste, and grill them over high heat. If the pork is thin and your fire is hot, this should take about 2 minutes per side.
Serve 2 pieces of pork per person, partially covered by a couple of tablespoons of the Chile-Coconut Sauce.