1. In a large frying pan, heat the oil over moderately low heat. Add the onion and cook, stirring occasionally, until translucent, about 5 minutes. Add the garlic and cook, stirring, 30 seconds longer. Stir in the tomatoes, red-pepper flakes, water, parsley, and salt. Reduce the heat and simmer, covered, for 10 minutes. Add the shrimp and cook, covered, just until the shrimp are pink, about 4 minutes.
2. In a large pot of boiling, salted water, cook the vermicelli until just done, about 9 minutes. Drain the pasta and toss with the tomato sauce and shrimp.
Variations: Lobster Fra Diavolo with Vermicelli: If you're in a celebratory mood, lobster would taste great in place of the shrimp.
Vermicelli Fra Diavolo: The full-flavored sauce can stand on its own without the shrimp. Add a sprinkling of Parmesan cheese.
Wine Recommendation: Look for a refreshing, fruity red to contrast with the heat of the red-pepper flakes. A bottle of food-friendly dolcetto or barbera from the Piedmont region of Italy would be perfect.