Bring a large pot of water to a boil. Make the dressing: In a bowl, whisk together all ingredients except oil, salt, and pepper. Whisking constantly, slowly drizzle in oil. Season with salt and pepper.
Make the salad: Add carrots, broccoli, and cauliflower to boiling water and cook until just tender, about 4 minutes. Drain and rinse under cold water.
In a large salad bowl, toss cooked vegetables, bell pepper, onion and cherry tomatoes with dressing. Cover and chill for at least 30 minutes. Toss again just before serving.