In a large bowl, combine cream cheese, sour cream, sugar, and vanilla.
Carefully fold in the grapes and gently toss until evenly coated. Meanwhile, in a small bowl, mix brown sugar and walnuts.
Sprinkle the brown sugar mixture on top of the grapes. Cover and chill at least 1 hour or overnight. Serve cold.
Video
Notes
If the cream cheese is not softened to room temperature, the cream cheese and sour cream won't combine well (the mixture will be lumpy). If this happens, stir it quickly with a whisk.