Bring a large pot of lightly salted water to a boil. Meanwhile, soak clams in cold water.
Add spaghetti to boiling water, and cook until slightly underdone; pasta will finish cooking in sauce. Meanwhile, place a large saucepan over medium-low heat, and add olive oil, garlic and chili pepper. Sauté gently, reducing heat if necessary so garlic does not brown.
Add vermouth and clams, and cover. Clams should open in about 2 minutes. (If pasta is ready first, drain it and toss with a small amount of olive oil.) Add hot drained pasta, cover, and shake pot gently. Allow to simmer for another 1 or 2 minutes until it is done to taste.
Discard any clams that have not opened. Add half the parsley, and shake pan to distribute evenly. Transfer to a plate or bowl, and sprinkle with remaining parsley