I too wondered how the St. Peter's Cross worked in a leg vise so I made one from wood as a prototype. It works great. Because the cross supports the outer leg as well as moves it in and out there is little to no load on the screw. To adjust the vise so the jaws close at the top first I just put a wedge in the track, works great. The vise has been on the bench for a few years and works fine. The problem I see with most vises is overclamping, all vises wrack to some degee. Learn to use what you have but don't be afraid to experiment. Just don't talk it to death.
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As you can see in the photo the jaws are open falrly wide and the fulcrum is still as high as a typical leg vise that uses a pin.