The vortices never provided all the lifting force. Up to about 6 or 7 deg AoA there was no vortex lift, just the usual wing tip vortices. Above that AoA the non-linear (vortex) lift grew steadily until at stall (about 23 deg AoA) the vortex lift was around 45% of the total.
I though delat wings didn't stall provided there was sufficient power to overcome the drag (thought at ludicrously high AoA that power requirement would be ginormouse!)?