Bring a large pot of salted water to a boil and add pasta; cook until al dente, about 10 minutes or as package label directs.
While pasta is cooking, whisk together vinegar, mustard, 1/4 tsp. salt and 1/8 tsp. pepper in a large bowl. Continue to whisk while adding oil in a slow stream. Dressing will thicken.
Drain pasta and add to bowl with dressing. Mix well. Stir in tomatoes, cucumber, onion, olives and feta. Season with salt and pepper. Just before serving, toss with parsley.
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