Bring 4 cups water and the vermouth to a boil in a large pot over high heat. Add the clams, cover and cook until they start opening, 4 to 10 minutes; as the clams open, transfer them to a bowl using tongs. (Discard any unopened clams.) Strain the cooking liquid through a fine-mesh sieve into another bowl; set aside.
Meanwhile, heat the olive oil in a large saucepan over medium heat. Add the bacon and garlic. Cook, stirring occasionally, until the bacon starts to crisp, about 7 minutes. Add the spring onions, sliced celery, 1/2 teaspoon salt and a few grinds of pepper. Cook, stirring occasionally, until the vegetables soften, about 7 minutes.
Sprinkle the flour over the vegetables in the pan; cook, stirring, until incorporated, about 1 minute. Slowly add the reserved clam cooking liquid, stirring and scraping up any browned bits from the bottom of the pan. Bring to a simmer and cook until thickened, about 3 minutes. Season with salt and pepper.
Add the clams to the saucepan (remove from the shells and chop, if desired); add the peas. Cook until warmed through, about 2 minutes. Remove from the heat and stir in the half-and-half. Top with celery leaves and serve with the baguette.