While the pasta is cooking, heat a large skillet over medium heat, melt the butter. Add the onions and cook, stirring frequently, until softened, for 4 to 5 minutes. Add in the garlic and toss for about a minute or two. Pour in the entire can of San Marzano peeled tomatoes and crush. Sprinkle in the crushed red pepper flakes and add the vodka and cook, stirring frequently, for about 5 to 6 minutes.