Vinaigrette: In a container, whisk together the vinegars, sugar, sweet mustard, salt and vanilla. Whisk in the oil. Makes a generous 1 cup of vinaigrette.
Salad: Arrange the romaine on individual salad plates.
To prepare the peeled and cored apples, cut each half into 1/4-inch thick slices, then cut each slice into thirds.
Place the apples in a small bowl and toss with enough of the vinaigrette to coat the pieces. You can prepare the apple mixture up to an hour ahead. Important: If you coat the apples with the dressing any longer than that, they will absorb too much of the vinaigrette, which overwhelms the apple flavor.
Divide the apple mixture among the salads. Top each serving with a portion of the cheese and then a portion of the hazelnuts.
Drizzle an extra bit of the vinaigrette over the greens and around the edges of the salad. Serve.
Alternatively: Instead of hearts of Romaine lettuce, consider a mixture of baby spinach, hearts of Romaine and/or other mixed young greens. You could also substitute feta or white cheddar, aged gouda, etc. for the blue cheese.
Another serving option would be to assemble the entire salad on a large platter instead of individual salad plates.