In a large wok, pour in enough vegetable oil to reach 3 inches (7.5 centimeters) high and place over medium heat.
Once thoroughly heated, gently add the potatoes and carrots. Cook, stirring occasionally, until golden on the edges, but not completely tender.
Use a slotted spoon to remove the potatoes and carrots from the wok and set aside in a large bowl.
Discard all but 2 tablespoons (30 milliliters) of the oil.
Pan-fry the marinated chicken in batches in the hot wok, adding more oil as needed. Cook on each side until golden, but not completely cooked through. Set aside on a plate and repeat with remaining chicken.
Pour in a little more oil if needed and when hot, add the sliced onion. Cook until softened and starting to turn golden.
Stir in the garlic and ginger. Cook until just fragrant, 30 seconds to 1 minute.
Mix in the curry powde and fry just until fragrant.
While stirring, pour in 1 1/2 cups (355 milliliters) of the chicken stock and 1 cup (237 milliliters) of the water. Add the browned chicken, lemongrass, Kaffir lime leaves, and a pinch of salt.
Bring the mixture to a boil, then reduce heat to a simmer and cook for 10 minutes, stirring occasionally.
Pour in the remaining 1 1/2 cups (355 milliliters) of the chicken stock and 1 cup (237 milliliters) of the water. Add the fried potatoes and carrots and continue to cook for another 5-10 minutes, until the vegetables are tender. If the vegetables were not fried first, the cooking time will be longer.
Stir in the coconut milk. Once the liquid starts to bubble, remove from heat. Adjust to taste with salt, freshly ground black pepper, and fish sauce. Discard the lemongrass and lime leaves.
Garnish with cilantro and serve with lime wedges and slices of Vietnamese baguettes.