Break up turkey bones; place in a large pot with onion, ginger, garlic, and red-pepper flakes. Add enough water to cover (10 to 12 cups); bring to a boil. Reduce heat to medium-low; simmer, partially covered, 30 minutes.
Discard large solids; strain broth through a fine-mesh sieve into a large saucepan (you should have 8 to 10 cups). Return to a boil. Add sliced cabbage to broth along with rice noodles, vinegar, and soy sauce.
Simmer until noodles are just tender, 2 to 3 minutes. Season with coarse salt. Serve garnished with cilantro sprigs and lime wedges, if desired.