Here’s something you can try. I tried this on my Spyderco with the full flat grind. I found I could shim it, but each time it came out to a different angle and I was looking for repeatability.
I noticed that there was a lot of flexibility in the position of the right (movable) side and decided that maybe the jaws could accommodate the angle difference. With both screws backed out and while holding the blade in position on the key pins, I pinched the top of the two vise halves together so that they were flat against both sides. This cants the bottom of the movable jaw out quite a bit. I snugged up the upper screw very lightly, then ran the lower screw all the way in to achieve the final clamp. This clamps the jaws tightly and uniformly against both flats of the blade as shown in this photo. My jaws normally read 13.3 degrees and in this case you can see the right jaw sits at 16.75 degrees.
Obviously, this setup puts the blade’s vertical centerline out of its normal vertical position. This knife has a 3.5 deg difference between the flats on one side and the other. With the jaws canted, the blade leans 1.75 degrees to the left, so I set the left sharpening angle 1.75 deg less than normal and the right angle 1.75 deg more than normal. I’m sure that this arrangement violates some law of the universe, but my knife doesn’t know the difference.
To speed things up, I’ll reverse my angle bar so the thumbscrews aren’t fighting the detents when I try to make half-degree adjustments without skewing the micro-adjusts for the ball joints.
I think the jaws will accommodate an angle difference of up about four and a half degrees. I’m hoping to stir up a hornet’s nest on this so someone can set me straight before lightning hits me.
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