In a large bowl of water, soak the vermicelli noodles as per the instructions on the packet; they will vary depending on the type of vermicelli. When al dente, drain in a fine meshed sieve.
Prepare the sauce in a small dish: pour in the fish sauce, dark soy sauce, oyster sauce, salt, black pepper, and sugar, and stir vigorously to combine well. Taste and adjust to suit your palate, then set aside.
In a large wok over high heat, heat 2 tablespoons soybean oil then fry half the garlic for a minute until fragrant, stirring constantly so it doesn’t brown. Add the pork and half the sauce and stir-fry for a couple of minutes or so; poke a digital kitchen thermometer into the largest piece; it will be ready at 63°C (145°F). Transfer the pork to a plate and set aside.
To the same wok over high heat, add the remaining 2 tablespoons soybean oil and fry the other half of the garlic for a minute until fragrant, stirring constantly, then add the mushrooms, and remaining sauce and stir-fry for a few minutes until the mushrooms begin to brown.
Add the drained noodles, return the pork to the wok, add half the sliced scallions, and stir-fry for a couple of minutes or so until any water has evaporated and everything is well combined. Taste, adjust the seasoning to suit your palate, transfer to a serving plate, and garnish with the remaining sliced scallions.