Wash and halve all the grapes. Pat them dry with a clean towel — excess water will make the dressing runny.
In a medium bowl, beat the softened cream cheese with a fork until smooth. Add the sour cream, granulated sugar, and vanilla. Stir until fully combined and creamy.
Add the grapes to the dressing and gently fold until every grape is coated. Cover and refrigerate for at least 1 hour to let the flavors meld.
Just before serving, toast the pecans in a dry skillet over medium heat for 2-3 minutes until fragrant. Stir constantly to prevent burning.
Sprinkle the brown sugar and toasted pecans over the top of the salad. Serve cold.