Preheat oven to 450° F. In a large bowl, whisk together the vinegar, oil, ketchup, orange juice, thyme, salt, and a few grinds of pepper. Add the meat or fish and coat well. Set the bowl aside, allowing the meat or fish to marinate for 15 minutes.
Arrange the meat or fish in a baking dish in a single layer; reserve the marinade. Bake 30 minutes for the pork, 30 minutes for the chicken, or 20 minutes for the salmon. Halfway through the cooking time, turn over the meat or fish and spoon the marinade over it, then return to the oven. Slice the meat and divide among 4 plates. Season with salt and pepper to taste.