Bring a large pot of water to a boil over high heat. Add salt when simmering.
Warm marinara in a saucepan over medium heat. Slowly add milk, stirring until combined. Bring to a simmer, then lower heat; simmer until sauce reduces slightly. Add crushed red pepper.
Add penne to boiling water. Cook until almost done; add vodka to marinara mixture, and raise heat slightly. Drain pasta, reserving 1/2 cup cooking water.
In a large bowl, toss pasta with sauce and Parmigiano-Reggiano, adding some reserved water to thin sauce, if needed. Garnish with parsley, and serve.