Preheat oven to 400 degrees. To make the spice rub, combine the oregano, garlic powder, and pepper in a small bowl.
Pat the pork dry with paper towels and rub all over with the spice rub. Place the pork in an oiled heavy roasting pan just large enough to hold it comfortably.
Arrange the tomatoes, eggplants, zucchini, bell pepper, and onion around the pork. Sprinkle with garlic, basil, and salt and pepper to taste. Scatter the capers over all.
Roast the pork until it is cooked through but still slightly pink in the center, about 1 1/2 hours. An instant-read thermometer inserted into the pork should read 155-160 degrees. Turn the vegetables twice during roasting.
Transfer the pork to a carving board, cover loosely with aluminum foil, and let stand for 10 minutes. Slice and arrange on a warmed platter. Surround with the vegetables and serve.