In a well-seasoned wok or large skillet heat oil and saute onion, garlic, mushrooms and chile until lightly browned. Add remaining sauce ingredients, stir well and bring to a boil. Pour into a bowl a nd set aside. Beat egg with fish sauce and sesame oil. Clean wok and reheat. Pour in eggs in a very thin layer and cook 10 minutes, turning several times. Turn out, roll up omelette and slice into 1/ 4-inch strips. Heat 1 tablespoon oil over high heat and saute pork for 2 minutes. Add scallions, bean sprouts, crab meat and egg strips. Sprinkle with sugar. Saute 30 seconds and transfer to a bowl.

Clean wok and heat remaining 2 cups oil over medium-high heat. In a large shopping bag tear vermicelli into bunches. Add one-eighth of noodles and fry 30 seconds. Remove to a warmed paper towel-lined platter and sprinkle with one-eighth of pork mixture. Repeat until all noodles and pork are used up. Pour oil from wok, add sauce mixture and reheat, stirring, until bubbly. Remove paper towels from platter, pour sauce over noodles and mix well. Serve immediately.