Add the lentils, thyme, bay leaf, pepper, and the remaining 2 teaspoons salt. Add the broth and water, and bring to a boil, skimming and discarding any foam as it rises to the surface. Reduce the heat and simmer until the lentils are tender, 15 to 20 minutes. (The cooking time depends on the age of the dried lentils.) Stir in the vinegar. Season to taste with salt and pepper. If needed, thin the soup with additional water or broth for the desired consistency. Serve in a bowl topped with fresh croutons, if using.