Change vertices using vertex shader

[xboct]

Hi guys,
i'm wondering, if i have a possibility to manipulate only a portion of the vertices form the vertex shader input register?
All examples I saw before, manipulate the whole input stream using one of the instructions defined in the Nvidia vertex shader specification. The thing I need is that only some of the vertices of my mesh have to be changed in the position.
Thanks.

[Mazy]

Well, you must have somekind of selection method even on the cpu.. so how do you know what vertices has to be changed, and are all information that you need to make that decision in the vertexdata you send to the card? if not, then add that data in another array, then all vertices can go through the same vertexprocessing, but you can create an 'if' statement by a linear intepolation kind of instruction serie

a = vertex*modelview*projectionView
b = vertex*specialstuff*modelview*projectionview
c = a switch variable that are 1 or 0

result = c*a + (1-c)*b