1) Fill a stockpot 3/4 full with salted water. When water comes to a rolling boil, add orzo. Cook until al dente, or according to package directions.
2) Drain orzo and cool on a baking sheet.
3) Make the vinaigrette. When orzo is at room temperature, toss with tomatoes and coriander. Dress with vinaigrette.
4) For the citrus vinaigrette: In medium bowl, whisk together lemon juice, mustard, zests, salt and pepper. Slowly whisk in olive oil until mixture has emulsified. Vinaigrette can be made up to 3 hrs ahead, covered and chilled. Bring to room temperature and stir before serving.