Wash the grapes and dry thoroughly with paper towels. Put the dry grapes in a 9-by-13-inch baking dish. In a bowl, combine the cream cheese, sour cream, granulated sugar and vanilla. Mix well using an electric mixer. Pour this mixture over the grapes and toss together until all the grapes are coated. Chill in the refrigerator.
Just before serving, sprinkle with the brown sugar and pecans.