Prepare the vinaigrette, just before use. Place the lemon in a small microwave-safe bowl and microwave until the essential oils in the skin are released. Since the lemon will be hot coming from the microwave, you will need to set it aside briefly, until it cools just enough to handle.
Pour vinegar into a blender, turn it on, and then squeeze in lemon juice from the microwaved lemon. Leaving the blender running, add 1 at a time through the feed opening: garlic, mustard, chives, basil, and yogurt, and blend until smooth. While keeping the blender running, add oil through the feed opening in a slow stream.
Mix together tomatoes, celery, and scallions and toss with dressing.