Preheat the oven to 425 degrees. Place fish in shallow baking dish, season lightly with salt and pepper and cover with buttermilk, turning to coat. Let soak while preparing the potatoes.
Cut the potatoes lengthwise into thick wedges (approximately 4 for smaller potatoes and 6 or 8 for larger ones). Add to a large pot and cover with cold water by 2 inches, along with the vinegar and a little salt. Bring to a boil over high heat then reduce to a simmer, and cook until just tender, about 10-15 minutes. Drain into colander and, when they look dry, toss with 2 tablespoons of olive oil, a pinch of salt, and black pepper. Place on a baking sheet and roast in the oven for about 30 minutes, or until crispy and browned.
Meanwhile, while the potatoes simmer, make the seasoned breadcrumb mixture by combining the panko, cornmeal, chili powder, paprika, thyme, ½ teaspoon salt, and a few grinds of black pepper. Put the breadcrumb mixture on a dinner plate for dredging. After putting the potatoes in the oven, remove the fish fillets from the buttermilk one-by-one, allowing some buttermilk to drip off, and coat both sides with the breadcrumb mixture. Place on a second baking sheet. Repeat until all fillets are coated. Drizzle the remaining 1 tablespoon olive oil over the fillets, and place in the oven once the potatoes have been roasting for about 15 minutes.
Start checking the fish for doneness after 15 minutes. It will feel firm to the touch, and if you peek inside the flesh will be opaque all the way through. Remove the fish and potatoes from the oven and serve warm with lemon wedges, malt vinegar, and a large green salad.