Combine the mushrooms and boiling water in a bowl; cover and let stand 15 minutes. Drain in a colander over a bowl, reserving 1 cup liquid. Discard stems; rinse and cut caps into thin strips.
Combine 1/2 cup reserved mushroom liquid and broth in a large saucepan; bring to a boil. Add the mushrooms, pork, and tofu; reduce heat, and simmer 3 minutes. Combine 1/2 cup reserved mushroom liquid, cornstarch, vinegar, soy sauce, fish sauce, and peppers in a small bowl. Add cornstarch mixture to saucepan; bring to a boil. Cook 1 minute, stirring constantly. Remove from heat; stir in onions and oil.