Put the coconut milk in a saucepan and add 1 cup water. Add the whites of the scallions and yellow onion and bring to a simmer over medium heat. Cook until onions have softened, about 10 minutes.
Add the broken pasta and cook 3-4 minutes; stir in the pork, rice vinegar, and soy sauce, and some pepper, and simmer until pork is cooked through, about 5 minutes. Add more hot water is soup is too thick. Taste and add more salt, pepper, and rice vinegar, if needed.
To serve, ladle into bowls and scatter the chopped scallion greens over the top.