Soak the clams in a large bowl of cold water, and sort through them, discarding any shells that remain open or are cracked or smashed.
Heat the oil in a large heavy-based pan that comes with a lid, then add the chopped shallot, stirring for a minute, grate in the garlic, and add the chilli flakes, stirring again over the heat so that it sizzles, though not long enough to let the garlic brown.
Stir in the tomato purée, then add the stock and the vermouth and let it come to the boil.
Add the fregola – it should be covered completely by the liquid – and let it simmer, still uncovered, for 10-12 minutes (or as instructed on the fregola packet).
Check that the fregola is nearly ready and then add the drained clams and cover the pan with the lid. Leave to cook for three minutes at a fast simmer, then uncover the pan to check that the clams have opened. Any clams that, once cooked, stay closed should be discarded.
Sprinkle in the chopped parsley and stir to let everything combine before ladling into four warmed bowls to serve, sprinkling with a little more chopped parsley as you go.