Stir together hoisin, sambal oelek, and 1 tablespoon vinegar. In another bowl, stir together bell pepper, bok-choy stems, and remaining 1/4 cup vinegar. Season with salt and pepper.

Place pork in a medium nonstick skillet over medium-high heat. Season with salt and cook, breaking up with the back of a spoon, until cooked and beginning to brown, 5 to 6 minutes (there will be some liquid in skillet). Add 1/4 cup hoisin mixture; cook until sauce is reduced and evenly coats meat, about 30 seconds.

Spread mayonnaise on buns. Serve pork in buns with bok-choy leaves, slaw, and herbs, with remaining hoisin mixture and sambal oelek alongside.