Bring lentils, water, and bay leaf to a boil in a 2- to 3-quart heavy saucepan, then reduce heat and simmer, covered, until almost tender, about 15 minutes. Stir in 1/2 teaspoon salt, then simmer lentils, covered, until tender but not falling apart, 3 to 5 minutes.
While lentils simmer, cook onion, carrots, celery, garlic, thyme, 1/4 teaspoon salt, and 1/8 teaspoon pepper in 2 tablespoons oil in a 12-inch heavy skillet over moderately low heat, stirring occasionally, until vegetables are just softened, 7 to 9 minutes.
Make vinaigrette by whisking together vinegar, mustard, and remaining 1/4 teaspoon salt and 1/8 teaspoon pepper in a bowl. Add remaining 1/2 cup oil in a slow stream, whisking until blended well.
Drain lentils in a colander, discarding bay leaf, and return to saucepan along with vegetable mixture and vinaigrette. Cook over low heat, stirring, until heated through. Keep warm, covered.