1. Make brine: In a large pot, bring 3 1/2 cups water to a boil. Stir in salt, brown sugar, and vanilla. Chill until cool.
2. Put pork in a 9- by 13-in. pan and pour on brine. Chill at least 3 hours and up to 12.
3. Make char siu glaze: In a small bowl, mix together ketchup, hoisin, sesame oil, garlic, ginger, and soy sauce. Pour half the sauce into another small bowl.
4. Prepare grill for indirect medium heat (350° to 450°; you can hold your hand 5 in. above cooking grate only 5 to 7 seconds). Lay pork over indirect-heat area and cook, covered, until meat reaches 135° on a meat thermometer, 15 to 20 minutes.
5. Using a pastry brush and one bowl of glaze, cover pork with glaze, saving 2 tbsp. for the pineapple. Cook pork (if using charcoal, add 6 to 8 briquets to maintain temperature), turning occasionally, until glaze has caramelized slightly and meat thermometer reaches 145°, 5 to 10 minutes. Transfer pork to a cutting board, tent with foil, and let rest 15 minutes.
6. Lay pineapple slices on direct-heat area of grill, brush with 2 tbsp. reserved glaze, and cook, turning once, until grill marks appear, about 4 minutes per side. Remove slices from grill and cut in half.
7. Cut pork into 1/2-in. slices. Cut a deep diagonal slit across the top of each roll. Fill each roll with a piece of pork, half a grilled pineapple slice, a cilantro sprig, and 1/2 tsp. glaze from second bowl. Serve rolls with remaining glaze for drizzling.
*Find aromatic Hawaiian vanilla extract at gourmet grocery stores and hawaiianvanilla.com; non-Hawaiian vanilla extract works too.
Make ahead: Brine pork and make char siu glaze up to 1 day ahead.
Note: Nutritional analysis is per serving.