Heat the vegetable oil in a large pot over medium heat. Add the pork cubes and brown them on all sides, about 6 minutes total.
Add the onion, garlic, and ginger to the pot. Cook for 3 minutes until the onion softens.
Stir in the fish sauce and brown sugar. Cook for 1 minute to coat the meat.
Pour in the chicken broth and add the bruised lemongrass stalks. Bring the mixture to a simmer.
Add the potatoes and carrots. Cover and cook for 25 minutes until the vegetables are tender.
Stir in the coconut milk and simmer uncovered for 10 minutes.
Remove the lemongrass stalks. Stir in the lime juice and serve with fresh cilantro on top.