In a large bowl, beat the sour cream, cream cheese, sugar, and vanilla until blended. Then pour the grapes into bowl and toss to coat all the grapes well with mixture.
Next, toast your pecan pieces until hot. Then add your brown sugar to pecans and toss well.
Now, sprinkle the brown sugar and pecans mixture on top of the grape mixture.
Cover and chill at least for 1 hour (or overnight). Serve cold.