On the line where you create the ellipse:
Dim aEllipse As New Ellipse(CenterPoint, axisvec, New Vector3d(0, 20, 0), 0.5, 0, Math.PI * 2)
You need to change the coordinates of the major axis like this:
Dim aEllipse As New Ellipse(CenterPoint, axisvec, New Vector3d(20, 0, 0), 0.5, 0, Math.PI * 2)