self.stretchy_spacer_thing =QSpacerItem(10,10,QSizePolicy.Minimum,QSizePolicy.Expanding)
self.some_Vbox_layout.addItem(self.stretchie)
# you can then take away the spacer item when you don't need it with .removeItem()
self.some_Vbox_layour.removeItem(self.stretchy_spacer_thing)