Sort through lentils to remove debris and shriveled beans, then rinse.
Add lentils to crock pot, along with barley, carrots, celery, onion, garlic, dried herbs and bay leaf.
Pour in broth, water, and tomatoes.
Cover and cook on low heat for 12 to 14 hours, or on high heat for 5 to 6 hours.
Discard bay leaf and just before serving stir in parsley and vinegar. (If you don't use the vinegar, the soup might taste more bland as you're omitting a major contributor of flavor.).
Lentils aren't like dried beans. They don't require soaking and cook pretty quickly when you do it right.
When you buy your lentils, go somewhere that has a decent turnover so your lentils haven't been sitting on the shelf for years. Also, don't add any additional salt to the soup while it is cooking, and don't add lentils to already boiling water as either one can make lentils tough.
