Combine the vinegar, lemon juice, oregano, and salt and pepper to taste in a small bowl and whisk together. Add the oil in a steady stream, whisking constantly.
Combine the lettuce, onion, bell pepper, tomato, cucumber, olives and feta in a large salad bowl. Pour the dressing over and toss until all of the ingredients are well coated. Taste and adjust the seasonings, adding more salt or lemon juice as desired.
Adapted from "One Dish at a Time: Delicious Recipes from My Italian-American Childhood and Beyond" by Valerie Bertinelli © Rodale 2012. Provided courtesy of Valerie Bertinelli. All rights reserved.