Mix salt and sugar together in a small bowl. Sprinkle all over ribs. Coat with red pepper flakes, five-spice powder, and black pepper. Slide ribs into a large vacuum-seal bag; pour in liquid smoke. Seal.
Immerse bag in a heat-proof container of water with a sous vide cooker. Set temperature to 140 degrees F (60 degrees C); cook for 8 hours.
Preheat oven to 450 degrees F (230 degrees C). Set oven rack about 6 inches from the heat source and turn on the broiler. Place a wire rack on top of a baking sheet.
Pull ribs out of the bag; spread onto the prepared baking sheet and brush with barbeque sauce.
Bake in the preheated oven until bubbly, 4 to 5 minutes. Flip and continue baking until other side bubbles, 2 to 3 minutes more.