Dried New Mexico red chiles, which offer a deep, mellow heat, are available at Latin American grocery stores and many supermarkets. Slideshow: More Curry Recipes
Rinse the dried chiles, then remove and discard the stems and seeds. Put the chiles in a small bowl and cover with warm water. Let soak for 30 minutes.
Lift the chiles from the soaking water, reserving 2 tablespoons of the water. Transfer the chiles to a blender, along with the chopped onion, garlic, ginger, curry powder and the reserved 2 tablespoons of water. Blend until very smooth.
Heat the vegetable oil in a large saucepan over moderately high heat. Add the chile paste and fry, stirring, until fragrant and a shade darker, about 5 minutes. Add the chicken broth, coconut milk and salt and bring to a simmer. Add the chicken breasts to the pan, reduce the heat to moderately low, and simmer until the chicken is just cooked through, about 10 minutes.
Using tongs, remove the chicken breasts from the pan and set aside on a plate until cool enough to handle, then tear the meat into bite-size pieces.
Meanwhile, cook the vermicelli in a large pot of boiling water, according to the package instructions. Drain and rinse under cold running water until cool. Set aside.
Stir the fish sauce, bean sprouts and red onion into the soup and simmer for 2 minutes. Stir the chicken back into the soup and simmer until heated through. Taste and adjust the seasonings.
To serve, divide the noodles evenly between 6 deep soup bowls and ladle the soup over. Garnish with chopped cilantro and serve with lime wedges.