Heat the vegetable oil in a large pot over medium heat.
Add minced garlic and cook until fragrant, about 1 minute.
Stir in the red curry paste and cook for an additional 1 minute.
Pour in the coconut milk and chicken or vegetable broth. Bring to a simmer.
Add the chicken slices, if using, and cook until they are cooked through, about 5-7 minutes.
Add the pre-soaked rice noodles and fish sauce, cooking for another 3-5 minutes until noodles are tender.
Stir in lime juice and adjust seasoning with more fish sauce if needed.
Serve the soup hot, garnished with chopped scallion, sliced red chili, and fresh cilantro.