Combine pepper, salt, and pink salt in a small bowl. Rub two-thirds of mixture evenly over surface of brisket. Reserve remaining one-third of mixture. Slice brisket in half crosswise in order to fit into large vacuum bags.
Place each brisket half in a vacuum bag. (Fold over the top of each bag while you add brisket so that no rub or juices get on the edges of bags, which can weaken the seal.) Add 4 drops (about 1/8 teaspoon) liquid smoke, if using, to each bag. Seal bags using a vacuum sealer and let rest for 2 to 3 hours in the refrigerator.
Set your sous vide to 68°C. Add brisket to water bath and cover it with aluminium foil. Cook for 36 hours. Allow cooked brisket to cool at least to room temperature before proceeding (an ice bath can speed up this process). Brisket can be stored in the refrigerator at this stage for up to 1 week before finishing.
Adjust oven rack to lower-middle position and preheat oven to 150°C. Remove brisket from sous vide bags and carefully blot dry with paper towels. (Liquid from bags can be added to your favourite bbq and simmered down to provide extra flavour.) Rub reserved salt-and-pepper mixture into surface of brisket. Place brisket on a wire rack set in a rimmed baking sheet, fat cap up, and place in oven. Roast until a deep, dark bark has formed, about 2 hours. Transfer brisket to a cutting board and tent with foil. Allow to rest until the temperature drops to between 63 and 74°C, about 30 minutes. Slice against the grain into thin strips and serve with white bread, dill pickles, and sliced onion.
