For best flavor, prepare the pasta salad the day before you plan to serve it.
In a large pot of boiling salted water, cook pasta until al dente, 9 to 10 minutes. Drain, and let cool slightly.
While pasta is cooking, stir together onion, vinegar, lemon juice, garlic, and oil. Season vinaigrette with 1 teaspoon salt, pepper, and sugar.
In a large bowl, combine pasta, bell pepper, cucumber, tomatoes, olives, capers, and vinaigrette. Cover and refrigerate at least 4 hours and up to 1 day.
Toss pasta with parsley and feta. Re-season, if desired, before serving.