Warm the oil in a large heavy-bottom pot over medium heat. Add the vegetables, lemongrass and curry leaves and saute, stirring occasionally, for about 5 minutes. Add the spices, salt and tomato paste and stir.
Add the stock or water and bring to a boil.
Simmer, uncovered, for 30 minutes, until the liquid reduced and the vegetables are tender. Add the fish sauce, soy or coconut milk and continue to cook for 3 more minutes. Taste and correct seasoning.
Cook the noodles according the package instructions. Make sure to wash them in cold water to stop their cooking.
Strain the soup into another pot, then with a fork or tongs grab the vegetables and put them back in the soup. Bring the soup into a boil. Remove from the heat and serve in medium bowls. Add a handful of noodles and garnish with cilantro. You can add sriracha for spiciness.