Don't throw that chuck of end cuts out, you just might have one of these old jigs floating around in that box of dear ol dads lapidary stuff he left in the garage. If not they still can be gotten a hold of.
Pretty simple to use; the above picture show the one side with the grab screws; adjustable plates and the tension screw.
You simply adjust those grab screw to where they will a good bite on the rock, those plates have multiple positions for those unusual shapes.
Once you get the rock positioned and snugged down, give her some real strong tugs, and I mean yank on that thing, make certain there is no give.
After your confident that your rock is not going to move or come lose in the clamp, your ready to place her in the vise, the block part of the clamp is what the vise tightens down to, not the plates, now your ready to get a few more slabs out of that once unable to vise chunk of rock; hope this helps and enjoy!