Beat 8 ounces cream cheese , 1 cup sour cream, 1/3 cup granulated sugar, and 1 teaspoon vanilla extract until smooth.
Wash and thoroughly dry 2 pounds green grapes and 2 pounds red grapes. Add them to the cream cheese mixture and fold into the dressing until they are evenly coated.
Cover with plastic wrap and refrigerate for at least 1 hour to overnight.
Wait until just before serving to sprinkle with 1/4 cup light brown sugar and 1/2 cup chopped pecans for the best crunch.
Notes
Be sure to wash and thoroughly dry the grapes before mixing so the dressing stays creamy and not watery.
Softened cream cheese blends much more smoothly than cold cream cheese.
Red and green grapes together give the best balance of sweet and tangy flavor, but you can use all one color if preferred.
This grape salad can be made up to 2 days ahead and stored covered in the refrigerator.