Set the pressure cooker on Sauté. Add the butter, onions, garlic, and crushed red pepper. Sauté for 3-5 minutes to soften the onions, stirring occasionally.
Add in the crushed tomatoes, water, vodka, penne, and 3/4 teaspoons salt. Stir to coat the pasta.
Cover and lock the lid into place. Set the Instant Pot on Pressure Cook High for 4 minutes. Once it is finished cooking, turn the Instant Pot Off, then press the button to Quick Release the steam, until the valve button drops.
Unlock the lid and stir the shrimp and cream into the pasta. Set the Instant Pot on Sauté and cook for 2-3 minutes, until the shrimp are pink. Taste, then salt and pepper as needed. Serve warm with Parmesan cheese, if desired.
STOVETOP INSTRUCTIONS: Boil the pasta as directed on the package and drain in a colander. Place the butter, onions, garlic, and crushed red pepper in a large sauté pan. Set over medium heat and sauté for 3-5 minutes. Add the crushed tomatoes, 1 cup water, and vodka to the pan. Simmer for 10 minutes. Pour the pasta, shrimp, and cream into the sauce and toss. Cook until the shrimp are pink.