Heat oil in a large skillet over medium-high heat. Add pancetta and cook 3 minutes, until golden brown. Add scallions, clams and the vermouth (if using) and simmer 1 minute. Add 1 cup of the broth and bring to a simmer. Cover and simmer 5 to 7 minutes, until shells open. Discard any shells that do not open.
With a slotted spoon, remove clams from pan and set aside. Simmer 2 minutes until sauce thickens. Season, to taste, with salt and black pepper. Serve 1/2 of the clams (24 to 36) over rice and cover with thickened sauce. Top with parsley and serve with the Garlic Toast. Reserve the remaining clams (about 2 dozen) for fettuccine, if desired.
Cut the baguette into 1-inch thick slices. Place the bread slices on a baking sheet and drizzle some olive oil over them. Bake for about 10 minutes, until golden and crispy. While still warm, rub the garlic clove on the toasts.