Caramel Grapes

In a large bowl, mix together sour cream, confectioner's sugar and vanilla. Stir in grapes. Set aside.
In a medium saucepan, combine butter and brown sugar. Stir constantly and cook over medium heat until mixture starts to thicken. It does not need to boil.
Pour brown sugar and butter over grape mixture and stir until well blended. Chill for at least 2 hours before serving.