Combine apple cider vinegar, ketchup, brown sugar, red pepper flakes, ground black pepper, and kosher salt in a small saucepan and bring just to boil; remove from heat and cool completely (can be made up to 5 days in advance).
Combine garlic, kosher salt, sugar, ground black pepper, cayenne, lemon juice, and mayonnaise in a wide bowl and puree with a hand blender. Fold in celery seeds and refrigerate overnight, or up to 5 days.
Fold the slaw dressing into the cabbage mix and let marinate for at least 2 hours or overnight.
Heat vegetable oil in a large pot or Dutch oven over medium heat. Carefully remove pork from the pouch, saving the jus. When oil is hot, place the pork into the pot and use a smaller lid to smash pork into pan, covering the entire bottom. Sear 4-5 minutes. Turn over as best you can and add the reserved stock from bag. When stock begins to simmer, turn heat down and pull pork apart with forks or tongs. Add the barbeque sauce. Return to simmer, then turn off heat and allow to rest for 5-10 minutes.
Toast buns if desired.
To serve, mound the meat onto bottom bun. Top with slaw and top with other half of bun.