In a large skillet over low heat, add the butter, onion, garlic and red pepper flakes. Cook, stirring occasionally, until softened, 5 to 10 minutes.
Increase the heat to medium-high and add the tomatoes, cream, vodka, tomato paste, ½ cup pasta cooking water and salt and pepper to taste. Let simmer for about 10 minutes, adding more reserved pasta water as needed.