Pound or blend the spice paste: put the shallots, garlic, ginger, galangal, fresh turmeric (if using fresh), candlenuts, coriander, white pepper, and 1 teaspoon salt in a mortar or small food processor and pound or process to a coarse paste. If using ground turmeric, stir it in at the end. Set aside.
Start the chicken in cold water: put the chicken in a large pot, add the 8 cups of water, bring to a boil over medium-high heat, skimming the foam, then lower to a gentle simmer.
Fry the paste until it turns nutty: heat the oil in a skillet over medium heat, add the spice paste, and fry, stirring often, for 5 to 7 minutes until fragrant, a shade darker, and slightly oily at the edges. Add a spoonful of hot chicken liquid if it sticks.
Season the broth: scrape the fried paste and all its oil into the simmering chicken pot. Add the lemongrass, lime leaves, and bay leaves. Simmer gently, partially covered, for 25 to 30 minutes until the chicken is cooked through and tender. Keep the broth at a lazy simmer, not a rolling boil.
Shred the chicken and finish the broth: lift the chicken out to cool briefly. Strain the broth if you like, then return it to the pot. Stir in the fish sauce and sugar, taste, and adjust with more salt or fish sauce a little at a time. Shred the chicken by hand, discarding skin and bones, and return it to the broth to warm through.
Prep the noodles and vegetables: soak the rice vermicelli in warm water for 15 to 20 minutes until pliable, then drain (or blanch 2 to 3 minutes if your package calls for it). Blanch the bean sprouts and cabbage together for 20 to 30 seconds until the sprouts just lose their raw snap, then drain well.
Build the bowls: divide the vermicelli among warm bowls, add the blanched sprouts and cabbage, shredded chicken, and a halved egg to each. Ladle about 1 1/2 cups of steaming hot broth over the top so the noodles and vegetables heat through.
Finish at the table: scatter fried shallots and celery leaves or scallions over each bowl. Set out lime wedges, extra fried shallots, and sambal so everyone can tune their own bowl. Serve right away, with steamed rice on the side if you like.