vb.net new point by coordinates

hey all,

i need to move a box to a new location by adjusting the z value. the Rhino.MoveObject function needs a start point and an end point. the start point is the centroid of the brep but I am having trouble with the end point. I used the PointCoordinates to get the location of the start point but i need to use those to create a new point with the same x and y coordinates but with a new z coordinate. 

In psudo code:

Dim box box = Rhino.GetObject("Select objects to move", , , True)

 If IsNull(box) Then Exit Sub
Dim start: start= Rhino.SurfaceVolumeCentroid(box)

Dim coord: coord= Rhino.PointCoordinate(start)

Randomize

Dim i :i= int((15*RND) -15)

If 15>i<15 Then print("true")

Dim last: last= Rhino.AddPoints(coord(0),coord(1),coord(2)+i)

Call Rhino.MoveObject(box,start,last)

Any ideas?