Season the swordfish with salt. Pour some flour on a plate and dredge the swordfish in the flour. Heat 3 tablespoons olive oil in a large Dutch oven over medium-high heat. Working in batches, cook the swordfish until golden on all sides, about 4 minutes. Transfer to a plate.
Reduce the heat to medium; add the remaining 2 tablespoons olive oil to the pot. Add the garlic, onion and red pepper flakes and cook about 3 minutes. Season with salt and add the wine. Cover and cook, stirring a few times, until the onion is tender, about 5 more minutes.
Add the seafood to the pot in layers, starting with the clams, then the shrimp, calamari rings and tentacles, and the swordfish. Cover and cook until the clams have just opened, about 6 minutes. Lightly stir, cover again and simmer 5 more minutes. Discard any clams that do not open. Stir in the butter, parsley, vinegar and salt to taste. Serve with bread.