Trim fat from beef. Cut across grain into thin slices; set aside.
Combine oyster sauce, soy sauce, and sugar. Stir well; set aside.
Bring 3 quarts of water to a simmer in a large Dutch oven. Add rice vermicelli; simmer 2 minutes or just until tender. Drain and return rice vermicelli to pan. Add coconut milk; toss well. Set aside; keep warm.
Heat oil in a wok or large nonstick skillet coated with with cooking spray over medium-high heat until hot. Add onion; stir-fry 2 minutes. Add beef; stir-fry 3 minutes. Add oyster sauce mixture; stir-fry 30 seconds. Remove from heat. Set aside; keep warm.
Combine bean sprouts, cucumber, cabbage, carrot, and basil in a large bowl; toss gently. Spoon bean sprout mixture into each of 4 large shallow bowls; top with beef mixture and 1 cup rice vermicelli mixture. Drizzle with Lime-Vinegar Sauce.