Preheat oven to 375°F.

Bring a large pot of salted water to a boil over high. Add ziti; cook according to package directions for al dente, about 10 minutes. Drain; rinse with cold water. Return pasta to pot; set aside.

Meanwhile, heat oil in a large Dutch oven over medium-high; add sausage, and cook, stirring occasionally to break into small pieces, until browned, 4 to 6 minutes.

Reduce heat to medium. Add onion and garlic; cook, stirring often, until softened, about 4 minutes.

Add tomato paste, and cook, stirring often, until slightly darkened in color, about 2 minutes.

Add vodka to sausage mixture; cook, scraping bottom of pan to loosen browned bits, until liquid is mostly reduced, 30 seconds to 1 minute.

Stir in marinara sauce and cream; bring to a simmer over medium-low. Simmer, stirring occasionally, until sauce is slightly thickened, about 10 minutes.

Remove from heat; stir in ½ cup Parmesan.

Stir 2 cups vodka sauce into drained pasta in pot until well combined.

Spread 3 cups vodka sauce in the bottom of a 9- x 13-inch baking dish.

Top with half of the ricotta (about ¾ cup), dolloping evenly over the sauce.

Top with half of the pasta mixture.

Dollop pasta mixture with remaining half of ricotta; sprinkle with 1 ½ cups mozzarella.

Top with remaining pasta mixture and remaining vodka sauce, spreading evenly.

Sprinkle with remaining 1 ½ cups mozzarella and ¼ cup Parmesan.

Bake in preheated oven until cheese is melted and browned in spots, about 30 minutes.

Top with parsley.