Whisk the hoisin sauce, vinegar, garlic and 1/2 teaspoon each salt and pepper in a large bowl. Add the pork and marinate 10 minutes.
Heat 1 tablespoon vegetable oil in a large skillet over high heat. Remove the pork from the marinade using tongs (reserve the marinade) and stir-fry until browned, about 4 minutes. Transfer the pork to a plate. Add 3 to 4 tablespoons water to the skillet, then pour the pan juices over the pork on the plate.
Add the remaining 1 tablespoon vegetable oil to the skillet; when hot, add the mushrooms and stir-fry until slightly golden, about 2 minutes. Add the coleslaw mix and cook until wilted, about 3 minutes. Add the pork, the reserved marinade and half of the scallions; stir-fry 2 more minutes. Season with salt and sprinkle with the remaining scallions. Serve the stir-fry in the lettuce leaves with more hoisin sauce.