In a medium saucepan, heat oil over medium. 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 5 minutes. Add lentils and cook until soup thickens slightly, 3 to 5 minutes. Stir in vinegar and season with salt and pepper.