Culling Vectors is changing data structure

Hello, I've searched the forums and have yet to find an answer to my problem.  I'm trying to seperate a list of vectors into two lists so that I can reassign their amplitudes.  Everytime I try to do this the vectors change directions.

I started with two surfaces (an inner and outer surface) that were contoured and then the contours divided into equal segments.  I then applied vectors from the contour points on the inner surface to their respective counterparts on the outer surface.  The lengths of the vectors are different based on the relationship of the inner and outer surfaces.  I then want to seperate the vectors into two groups.  The first group will be vectors up to a certain length and the second group will be all vectors longer than that length.  I then  reassign the amplitude of all the vectors in the first group to a predefined integer and all the vectors in the second group will have an adjustable amplitude based on a formula.  The next step is to translate the points from the inner surface by the amount and direction of all the vectors and then draw lines between the inner surface points and the newly translated points.  The end result will be a vertical loft between associated lines on each contour. 

The problem I have is that everytime I try to use cull or dispatch on the vectors to seperate them into groups, the vectors change directions on me.  I believe the problem is that when I use cull or dispatch, it changes the index of the vector list and the inner surface points no longer maintain the same relationship with the outer surface points, thus changing the data structure and respectively the vector direction.  Is my thinking correct?  Does anyone know a work around for this?

Thank you in advance.