Fill a large pasta pot with a strainer with salted water and bring to a boil over medium-high heat.
Heat the oil in a large skillet over medium-high heat. Add the chorizo and cook until lightly browned and cooked through, 6 to 8 minutes, breaking up the meat with a rubber spatula or wooden spoon. Use a slotted spoon to transfer the chorizo to a bowl. Add the onions and a large pinch of salt to the skillet and cook, stirring frequently, until the onions are translucent and soft, 4 to 5 minutes. Add the chorizo back to the pan, stir in the vinegar and cook for 1 minute, stirring constantly. Remove from the heat and set aside.
Melt the butter in a large Dutch oven over medium heat. Whisk in the flour and cook, whisking constantly, until smooth and the flour starts to smell nutty, about 3 minutes. Slowly whisk in the cream until smooth. Whisk in a large pinch of salt and a couple grinds of black pepper. Bring to a low simmer, adjust the heat and cook, whisking frequently, until the mixture is the consistency of thick gravy, about 5 minutes.
Meanwhile, cook the elbow macaroni in the boiling water according to the package directions for al dente pasta. Drain the macaroni, reserving 1 cup of the cooking water.
When the cream mixture is thickened, turn off the heat and add the Brie. Next add the Cheddar and Parmesan, hot pasta and chorizo mixture to the Dutch oven. Stir to combine, adding the pasta water as needed to thin out the sauce. Season with additional salt and pepper if needed. Scoop into 6 to 8 bowls and garnish with the parsley and scallions.