Add the stock to a medium pot. Remove the dry outer layers of the lemongrass, cut into 3-inch lengths, and halve lengthwise. Add the lemongrass, cilantro roots/stems, and fish balls to the stock. Bring to a simmer, cover, and allow to simmer while you prepare the other ingredients.
Make the chili vinegar by combining the Thai chilies and white vinegar. Set aside for at least 15 minutes, or refrigerate and keep it around for use for several weeks!
Bring a large pot of water to a boil. Add the ground pork, and cook until opaque. Remove and divide between two bowls. Then boil the noodles until al dente.
While the noodles are cooking, add the following seasonings to each bowl: 2 teaspoons sugar, 1 tablespoon fish sauce, 1 1/2 tablespoons fresh lime juice, 1/2 to 1 teaspoon dried chili flakes, 1 tablespoon chopped peanuts, 1 tablespoon chopped scallion, and 1 tablespoon chopped cilantro.
Add the noodles to the bowl, and ladle over the pork broth and fish balls. Top with garlic oil or lard, if using. Mix up your noodles and enjoy!
Notes
To make garlic oil, finely chop 7-8 cloves of garlic, and fry over medium heat in ¼ cup oil until the garlic just turns a pale golden color. Remove from the heat.