Flake the salmon into a large bowl. Using a fork, mix in 1/2 cup panko, the scallion whites, the celery, the parsley, 2/3 cup mayonnaise, 1/2 teaspoon salt, a pinch black pepper and the cayenne. Refrigerate for 30 minutes.
Form the mixture into 8 patties about 1 inch thick. In a bowl, mix the remaining 2 1/2 cups panko with 1 teaspoon salt. Coat the patties with the panko, then the eggs, then the panko again.
In a heavy skillet, heat half of the olive oil over medium heat. Add 4 salmon patties and cook until golden, about 3 minutes per side. Drain on paper towels. Repeat with the remaining oil and patties.
Combine the remaining 1/2 cup mayonnaise, the vinegar and 1/3 cup scallion greens; season with salt and pepper. Serve 2 cakes per person with the sauce alongside.