Heat the oil in a large stock pot over medium-high heat. Add the onions and carrots and sauté for 5 minutes. Add the garlic and cook, stirring, for another 30 seconds. Add lentils, broth, salt, and pepper and bring to a boil. Reduce heat, put the lid on the pot, and simmer for 35-40 minutes or until the lentils are tender.
Remove half of the soup from the pot, blend in a blender until smooth, and return to the pot. Heat for a few minutes, stir in the vinegar, and serve.