For the vinaigrette: In a small bowl, whisk together the vinegar and salt. The salt crystals should dissolve. Whisk in the mustard, oregano, and black pepper. Slowly whisk in the olive oil to form a smooth emulsion. Crumble in the blue cheese, if using. Pour the vinaigrette into a serving bowl to be passed at the table with a spoon. Refrigerate until ready to serve.
For the salad: Set out 8 salad bowls, preferably clear glass so the guests can see the layers of vegetables. For each bowl, put down a layer of arugula and then a layer of fennel. Season with salt and pepper, to taste, and sprinkle 1 tablespoon Parmesan and 1 tablespoon Prosciutto Bits over the fennel. Next, put down a layer of zucchini and a layer of red cabbage. Once again, season with salt and pepper to taste and sprinkle with Parmesan and Prosciutto Bits. Put down a layer of carrots over the top and finish with a small sprinkling of Parmesan and Prosciutto Bits.
Serve the salads at room temperature. Pass the vinaigrette at the table. Allow guests to dress their own salads and mix up the layers.