Whisk the hoisin, vinegar, garlic and 1/2 teaspoon each salt and pepper in a large bowl. Add the pork and marinate 10 minutes. Remove the pork from the marinade using tongs(reserve the marinade) and stir fry until browned. Transfer the pork to a plate. Add 3-4 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 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 rhe remailning scallions. Serve the stir fry on the lettuce leaves with more hoisin sauce.