In medium saucepan, combine lentils and water. Bring to a boil. Reduce heat; cover and simmer 10 minutes.
Meanwhile, in 3 1/2- to 4-quart slow cooker, combine celery, carrots, onions, ketchup, brown sugar and Worcestershire sauce; mix well. Stir in lentils with water.
Cover; cook on low setting for 10 to 12 hours. Stir in vinegar. Spoon 1/2 cup filling into each toasted bun.