There have been jigs around for eons to slip into the one side of a vise to prevent it racking. Such as this rather nice version from Veritas ...
I was going to write that it is one more jig than I want to have lying under my bench, but I have several lying there already! Really what I just wanted to do was build an anti-rack jig into the leg vise.
This is what I mean by racking - you place a board into the one side of the vise, and it looks like this ...
That is not good for the vise, and the vise does not hold the board securely.
Under my bench is a jig that is simply a bolt that can screw outwards towards the chop. This is attached to the bench leg with an L-bracket ...
Place the board at the "other" side of the vise, and lightly close it down ...
Now screw the bolt out until it hits the chop (the chop has a metal insert in its face - one can countersink a bolt head - to prevent harm to the chop) ...
Now you can work on the other side of the vise without any racking. Remember to retract the bolt when finished.
Regards from Perth
Derek