Place pork neck bones and spareribs in a stockpot with water to cover. Add salt and bring to a boil, skimming the fat off the surface of the broth. Reduce heat; add onions, tomatoes, and taro. Boil until pork meat is very tender, 30 to 40 minutes.
Stir lemon juice and fish sauce into the broth. Continue boiling until flavors come together, about 15 minutes. Add bok choy, leek, and shrimp. Cover, reduce heat, simmer until shrimp are opaque and bok choy is tender, about 10 minutes more.