With a hand mixer, mix cream cheese, sour cream, sugar, and vanilla until smooth.
Gently stir grapes into the cream cheese mixture. Spoon into a large serving dish. Allow the salad to chill in the fridge for an hour. The salad can be kept in the fridge a day before serving if needed.
Just prior to serving the salad, mix the pecans and brown sugar together in a small bowl. Sprinkle pecan mixture over the salad and serve.