Hello,
the object is created into the layer specified by ObjLayer[].
So, your code should be:
ImageEnVect1.LayersAdd ();
ImageEnVect1.ObjLayer[-1] := ImageEnVect1.LayersCurrent;
ImageEnVect1.IEBitmap.Fill(clwhite);
ImageEnVect1.MouseInteractVt := [miPutPolyline];
If you want create the object into currently selected layer then you have also to change ObjLayer[] whenever current layer changes. This can be done, for example, handling OnLayerNotify event:
procedure TForm1.ImageEnVect1LayerNotify(Sender: TObject; layer: Integer; event: TIELayerEvent);
begin
if event = ielSelected then
ImageEnVect1.ObjLayer[-1] := layer;
end;