Place the vermicelli in a large bowl and cover with hot water. Soak for 15 minutes. Soak bean sprouts in cold water. Drain just before using.
Heat the oil in a large frypan over medium-high heat. Add the garlic, and fry until fragrant. Add the prawns; cook and stir for about 3 minutes. Mix in the sugar, fish sauce and tomato sauce. Pour in the eggs, and stir for a minute. Add the noodles; stir-fry until they are coated with the sauce. Mix in half of the bean sprouts, and fry until soft.
Place on a serving plate. Set remaining bean sprouts, coriander and lemon wedges on a separate plate to add to individual servings as desired. Sprinkle the noodles with dried prawns, peanuts, and chilli powder. Top with spring onion.