Peel and devein the shrimp, reserving the shells. Set the peeled shrimp aside. Combine the vegetable stock and reserved shrimp shells in a saucepan. Simmer together for 30 minutes. Then strain out and discard the shrimp shells, reserving the stock.

Heat a stockpot or Dutch oven over medium heat. Add the vegetable oil and lightly sauté the onion, carrot and celery until tender – about 6 to 8 minutes. Add the garlic, ginger, chili pepper halves and cook for 1 to 2 minutes. Add the reserved vegetable stock and coconut milk, and bring to a simmer. Simmer for 20 minutes.

Meanwhile, in a separate bowl or saucepan, pour boiling water over the rice noodles and let them sit while the soup simmers.

Drain and add the noodles to the soup, along with the shrimp, soy sauce and scallions. Simmer just until the shrimp is pink and opaque – about 2 to 4 minutes. Add the cilantro and serve with the lime wedges.