Soak the vermicelli in hot tap water for 8 to 10 minutes until pliable but still firm, then drain well. Do not boil it; the noodles finish cooking in the wok and boiled vermicelli turns to mush.
Mix the curry powder, turmeric, soy sauce, sugar, and white pepper with 3 tbsp water to make a loose paste. Pre-mixing means no raw spice pockets later.
Heat 1 tbsp oil in a wok over high heat. Stir-fry the chicken until just cooked, about 3 minutes, and set aside. Add the prawns, 90 seconds per side until pink, and set aside with the chicken.
Add 1/2 tbsp oil and pour in the eggs. Let them set halfway, then scramble into ribbons and push to the side of the wok.
Add the last 1 tbsp oil with the onion and bell pepper and stir-fry 2 minutes. Add the drained noodles and the curry mixture and toss constantly with tongs or chopsticks for 2 to 3 minutes until every strand is evenly yellow.
Return the chicken and prawns, add the bean sprouts and scallions, and toss for a final minute. The sprouts should stay crunchy. Taste; it usually wants a last splash of soy.