Prepare the brine: Combine the salt, brown sugar, water, and vodka, if using, in a large shallow container such as a roasting pan. Add the whole gutted skin on whitefish, then cover and refrigerate for 8 to 12 hours. (If desired, keep the fish submerged in the brine by topping it with resealable bags of ice.).
Remove the fish from the brine and rinse thoroughly, inside and out, under cold running water. Pat dry with paper towels. (Discard the brine.).
When ready to cook, start your smoker and establish the lowest heat you can. (about 180 degrees.) Place the fish on the grill grate. Tuck 2 or 3 balls of foil in the cavity so the interior gets more exposure to the smoke. Smoke the fish for 3 to 4 hours, or until it flakes easily when pressed with a fork and has an internal temperature of 150 degrees F.
Let cool, then wrap in plastic wrap and refrigerate.
Make the salad: Remove the skin and break the fish into flakes; remove any pin bones you find. (There are a lot of them!) Add the celery, red onion, lemon juice, dill, mayonnaise, sour cream, and pepper. Stir gently with a rubber spatula to combine.
To serve, mound the salad into leaves of lettuce or radicchio, if desired, and accompany with bagels or bagel chips, crackers, or cocktail breads. Garnish with sprigs of fresh dill.