Mix vinegar, oil, sugar, garlic powder, salt and desired amount of parsley herb grinder in medium bowl with wire whisk until well blended. Set aside.
Bring stock to boil with desired amount of parsley herb grinder in small saucepan on medium-high heat. Stir in couscous. Cover. Remove from heat. Let stand 5 minutes or until liquid is absorbed. Fluff with fork.
Spoon couscous into bowl with the vinaigrette. Add tomatoes and onion; mix well. Refrigerate 1 hour or until ready to serve. Season to taste with additional parsley herb grinder.