float cone_angle = (float)(R.NextDouble() * angleDiff);
float roll = (float)(R.NextDouble() * 360.0);

Vector3 ray = reflectedRay.Direction;

Vector3 normal =
    ray.X == 0 ? new Vector3(1, 0, 0) :
    ray.Y == 0 ? new Vector3(0, 1, 0) :
    ray.Z == 0 ? new Vector3(0, 0, 1) : new Vector3(-ray.Y, ray.X, 0);

Matrix rot_cone = Matrix.CreateFromAxisAngle(normal, MathHelper.ToRadians(cone_angle));
Matrix rot360 = Matrix.CreateFromAxisAngle(ray, MathHelper.ToRadians(roll));
Matrix trans = Matrix.CreateTranslation(ray);
Matrix itrans = Matrix.CreateTranslation(-ray);

Vector3 result = Vector3.Transform(ray, (rot360  * rot_cone));