Bring a large pot of salted water to a boil. Cook the pasta according to al dente package instruction, 11 to 13 minutes. Reserve ½ cup pasta water and drain pasta.
In a large skillet, large enough to hold all of the cooked pasta, over medium heat add olive oil, onions and garlic. Sautee until translucent, about 3 minutes.
Add the tomatoes, vodka, and season with salt and pepper. Simmer for 10 minutes until alcohol burns off. Stir in heavy cream and butter.
Add the pasta to the skillet with the sauce. Toss the pasta in the sauce over high heat until well coated. Add a little of the cooking water if the sauce is too thick.
Top with Parmesan and parsley and mix gently.