Rub the roast with the salt and pepper. Place it in a resealable plastic bag. Add the parsley, 2 tablespoons of thyme, and the garlic and pour in the vodka and olive oil. Seal the bag and place it in a bowl in the refrigerator for 2 to 3 days, turning the bag twice a day so the meat marinates evenly.

Let the meat stand at room temperature for 1 to 2 hours before cooking.

Preheat the oven to 425°F.

Take the meat out of the plastic bag; reserve the marinade. Heat a large nonstick skillet over high heat. Add the vegetable oil and heat until hot. Sear the roast on all sides, about 4 to 6 minutes.

Transfer the meat to a baking pan. Insert a meat thermometer into the thickest part of the roast. Roast in the middle of the oven, turning once, for about 1½  hours, until the thermometer (or an instant-read thermometer) registers 130°F for medium-rare; before the roast is done, pour the reserved marinade over it. Transfer the meat to a cutting board and let it rest, uncovered, for at least 20 minutes. Set the baking dish aside.

Just before serving, pour the cooking juices into a saucepan and heat gently. Stir in the butter. Season it with salt, pepper and the remaining thyme. Strain the sauce and discard the herbs.

Carve the meat and arrange on a platter with the sauce on the side.