This is all very hifalutin and everything, but the vortex is surely a consequence of the air being deflected -- it gets twisted and bent out of shape as we all do when our preferred path is obstructed.
If you want to calculate that lift to the thousandth decimal place, maybe you need an ultra-refined method, but for me . . . if my car hits the guardrail, it will bounce back into the road and the guardrail will get dented . . . and Newton is happy.
Brian Hancock gets into what I guess is the Gentry theory in his book, Maximum Sail Power. Very nice for fluid dynamicists but for those of us without microscopes . . . not very helpful