Rinse pork and cut meat in half. Place halves in a 4 1/2-quart or larger electric slow-cooker. Sprinkle with garlic, five spice, and sugar. Add soy sauce and 1 cup water.
Cover and cook until pork is very tender when pierced, 7 1/2 to 8 1/2 hours on low, 5 1/2 to 6 1/2 hours on high. If possible, turn meat over halfway through cooking. With 2 slotted spoons, transfer pork to a 9- by 13-inch pan.
Bake in a 450° oven until well browned, 15 to 20 minutes. Using 2 forks, separate pork into shreds; discard bones and fat. Put meat in a bowl, cover, and keep warm.
As pork bakes, skim and discard fat from cooking liquid; pour into a 2- to 3-quart pan. Add vinegar. Bring to a boil over high heat; stir often until reduced to 1 cup, about 15 minutes.
Stack tortillas and seal in foil. Add to oven and bake until tortillas are warm in the center, 10 to 12 minutes. Place in a towel-lined basket.
Pour reduced sauce over shredded pork. Spoon some of the meat onto a warm tortilla; top with cabbage, onions, cilantro, and hoisin to taste. Fold 1 end of tortilla over filling, overlap sides to enclose, and hold shut to eat.