In three separate bowls of hot water, soak the noodles, dried shrimp, and mushrooms until each has softened, about 20 minutes. Drain the shrimp, discarding their soaking liquid, and set aside. Use a slotted spoon to remove the mushrooms (save their soaking liquid for another use), and squeeze them to extract any excess water. Trim and discard the mushroom stems; julienne their caps, and set aside.

In a small pot over medium heat, heat the oil. Add the garlic and cook, stirring occasionally, just until golden, 2–3 minutes. Immediately remove the pan from the heat and pour the garlic oil into a small, heatproof serving bowl.

To a large pot, add the rehydrated shrimp, julienned mushrooms, stock, crushed scallions, and ginger. Season lightly with salt, then bring to a boil over high heat. Turn down the heat to medium-low and continue to cook at a simmer until the aromatics are softened and the broth is rich and flavorful, about 15 minutes.

Meanwhile, bring a second large pot of water to a boil over high heat. Drain the vermicelli, add it to the boiling water, and cook for 5 seconds; remove the noodles with a spider skimmer or long-handled sieve and divide among 4 large soup bowls. Add the sliced chicken to the boiling water, cook for 10 seconds, remove with the sieve, and divide among the bowls. Add the bok choy to the pot and blanch until just tender, 1–2 minutes. Remove the bok choy and arrange on top of the chicken. Ladle about 2 cups of the broth into each bowl, then garnish each serving with sliced scallions, cilantro, and chiles. Drizzle with the garlic oil just before serving.