To keep the dip vegetarian, make sure you are using vegetarian Worcestershire sauce. Also keep an eye out for gelatin in reduced fat/fat free dairy products.
Serving size is a generous ¼ c (12 total servings). If you have a large spread of appetizers, a more realistic serving size would be 2 tbsp. per person (24 servings at about 50 calories each). Nutritional estimate uses fat free yogurt, fat free sour cream, and full fat mayo.