In a large bowl, combine the vinegar, olive oil, oregano, and sugar, whisking together until the sugar is dissolved.
Toss the carrots, onion, jalapeño, and garlic in the dressing. Cover and leave at room temperature to marinate while you prepare the lentils.
Bring 3 cups of water to a boil in a medium saucepan. Add a pinch of salt and a dried bay leaf. Add the lentils and return to a boil. Reduce heat to medium and cook until the lentils are tender, but still holding their shape. This should take about 25 minutes. Near the end of the cooking time, the water level may get low. Add more water as needed. Drain the cooked lentils well and let cool to room temperature.
Once cool, add the lentils to the marinated vegetables and stir to coat. Add salt and pepper to taste. Eat immediately or allow the salad to marinate in the fridge.