In a medium saucepan, heat oil over medium heat. Add onion, carrot, celery, and garlic; season with salt and pepper and cook, stirring occasionally, until onion softens, 3 to 5 minutes.
Add broth, bring to a boil, and cook for 5 minutes.
Add lentils and cook until soup thickens slightly, 3 to 5 minutes. Stir in vinegar and season with salt and pepper.
