Boil the chicken breasts until tender and cooked. When cool enough to handle shred with a fork or with your hands. Retain the liquid where it was boiled (if not using chicken broth).
Boil enough water for the pasta. Cook the macaroni al dente (not mushy, still retaining some bite). While waiting for the pasta water to boil, prepare the vegetables.
In a large pan, heat the oil. Saute the onions, garlic, celery, and carrots until the onions become translucent and the vegetables are tender about 5 minutes. Season with a bit of salt and pepper.
Stir in the shredded chicken. Add the broth (or water). Bring to a boil. Pour the evaporated milk, add the chopped cabbage, and season with fish sauce. Cook for another 2-3 minutes.