How Rotate a vector along axis of its plane??

I have a curve which I am dividing and than making a sine wave out of it by moving the points along its vectors with a sin wave attractor point.

At the moment the vectors that I am getting are perpendicular. I need a horizontal vector exactly 90 degree to that of perpendicular to make a sine wave at the bottom most part of the ridge. 

The idea is to have a sine wave on the top and a sine wave at the bottom and loft them to make an undulating surface..

The bottom surface is a plane. I have tried rotating the vector by using the same unit vector on the top and rotating it by 90 degree but the thing doesnt work.

Any suggestions on how to get this solution? 

(Plane is used to get curve >which is divided to get point> The points are than moved by sine wave attractor> How to get the vector exactly in the xy direction of plane not the world.)

The definition is made to make series of such porous patches on mesh. So I dont want a world rotation axis but one which is taken from the plane as shown below.