Wash all grapes under cold running water. Pat completely dry with a kitchen towel, remove any remaining stems, and let air out for a few minutes.
Beat softened cream cheese in a large bowl until completely smooth and lump-free, about 1 to 2 minutes.
Add sour cream, granulated sugar, and vanilla extract. Mix until the dressing is uniform and creamy.
Fold in all grapes with a rubber spatula, turning gently from the bottom until every grape is coated.
Transfer to a serving bowl, cover with plastic wrap, and refrigerate for at least 1 hour.
Just before serving, combine brown sugar and pecans. Sprinkle evenly over the chilled salad.
Serve within 10 minutes of adding the topping.