In a large bowl, combine sour cream, Greek yogurt, sugar and vanilla. Add grapes to mixture and gently stir until all grapes are coated. In a small bowl combine the pecans and brown sugar. Add the pecan crunch to the bowl of grapes but reserve 1/4 of the mixture. Transfer to serving dish and sprinkle the remainder of the pecan crunch on the top. Cover and refrigerate for 2-3 hours prior to serving.