Heat vegetable oil in a saucepan over medium heat. Add chicken and cook until brown, about 10 minutes. Stir in lemongrass, onion, chicken stock, chicken bouillon, and garlic. Bring to a boil; reduce heat and simmer for 5 minutes. Add coconut milk, potatoes, carrots, and curry powder. Simmer until potatoes and carrots are tender, about 20 minutes. Stir in bell pepper, peanut butter, salt, and pepper.