Discard any broken-shell or open (dead) clams. Place remaining clams in large container. Cover with 1 1/2 cups water and the vinegar. Let stand 30 minutes; drain. Scrub clams in cold water.
Meanwhile, discard any broken-shell or open (dead) mussels. Scrub remaining mussels in cold water, removing any barnacles with a dull paring knife. Pull beard by giving it a tug (using a kitchen towel may help). If you have trouble removing it, use a pliers to grip and pull gently. Place mussels in large container. Cover with cool water. Agitate water with hand, then drain and discard water. Repeat several times until water runs clear; drain.
Heat oven to 350°F. In 4-quart Dutch oven, mix clam juice, tomatoes, tomato sauce, wine and 1/2 cup of the pesto. Layer cod, shrimp, scallops, mussels and clams in Dutch oven. Heat to boiling over medium-high heat; reduce heat. Cover and simmer 15 to 20 minutes or until mussel and clam shells have opened.
Meanwhile, in small bowl, mix butter and remaining pesto until well blended. Spread on both sides of bread. On ungreased cookie sheet, place bread in single layer. Bake 10 to 15 minutes, turning once, until toasted on both sides.
Discard any mussels or clams that don’t open. Spoon stew into soup bowls; top with croutons.