Whisk together vinegar, mustard, salt and black pepper in small bowl. While whisking, slowly drizzle in olive oil, continuing to whisk until the mixture is well blended and thick. Whisk in Italian seasoning. Cover; refrigerate until ready to use.
Place peppers in glass microwave-safe dish. Add enough cold water to depth of about 1/8 inch (cook in batches if necessary). Cover dish tightly with plastic wrap. Microwave on 100 percent power for 6 minutes or until peppers are softened slightly but still somewhat firm. (Or simmer in water in small saucepan.) Remove peppers from dish. Let cool. Pat dry with paper towels.
Mix together the orzo, tomatoes, and olives in a large bowl. Stir in chicken and feta cheese. Pour on 3/4 cup of the dressing; gently fold together with a rubber spatula until all the ingredients are evenly coated with the dressing. Drizzle on the remaining dressing, if desired.
To serve, fill each pepper half with about 1-1/3 cups of the orzo filling. Garnish with the chopped parsley, if desired. If pepper halves are not large enough to hold all the filling, serve remainder of filling on the side. Makes 8 servings.