Heat the vegetable oil in a small saucepot, add the onion and garlic and saute for a minute or two. Add the curry paste and the sugar and saute another minute. Stir in the coconut milk, lime juice, rice wine vinegar, and chicken broth. Bring the liquid to a simmer and cook for about ten minutes. Stir in the peanut butter and cook another 5 minutes. Transfer the sauce to a blender and process until smooth. Serve warm.