Heat olive oil in a large pot over medium heat. Add leeks and onion; cook and stir until softened, 8 to 10 minutes. Add fennel and garlic; cook and stir until fragrant, about 5 minutes.
Pour fish stock and vermouth into the pot. Add tomatoes, red bell pepper, orange zest, red pepper flakes, pepper, and bay leaf. Bring to a boil; reduce heat, cover, and simmer until red bell pepper is soft, about 20 minutes.
Bring stock back to a boil. Add cod fillets; cook for 1 minute. Add shrimp, clams, and mussels; cook, shaking pot occasionally, until shells open and shrimp turn opaque, 1 to 2 minutes.