In a medium saucepan, bring the chicken broth, garlic, cumin and chili powder to a boil over medium-high heat.
Meanwhile, coarsely chop the tomatoes, scallions and bell pepper. Break the vermicelli into 2-inch pieces.
Add the chicken to the boiling broth, reduce the heat to medium-low, cover and simmer until the chicken is cooked through, about 10 minutes. Remove the chicken and set it aside to cool slightly.
In a large nonstick skillet, warm the oil over medium-high heat until hot but not smoking. Add the vermicelli and cook, stirring, until the vermicelli is lightly browned, about 2 minutes.
Add the broth mixture, tomatoes, scallions, bell pepper, green chilies and salt, and bring to a boil over medium-high heat. Reduce the heat to low, cover and simmer, stirring occasionally, until the pasta is cooked, about 7 minutes.
Meanwhile, shred the chicken. Mince the cilantro (if using).
Stir the shredded chicken and cilantro into the pasta mixture and serve hot.