If yours is a double-jawed vise, consider adding a batten to the bottom edge of one inner jaw. Do this and the top lips bite more tightly to really grip your saw plate. Barry Bate showed us this at Sheffield Sawfest 2018 and it is a cracking good idea. He glued wooden coffee stirring sticks approx 2 mm thick onto the lower edge of his panel and tenon saw vise. I find 12 mm thick ply works well on this pitsaw vise. Give it a try – I think you’ll be very pleased.
Tightening the eyebolt at location 1 generates extra force on the upper jaw at location 2. For less than 10 minutes work I now have a really great vise.
Observe the result of the forces along the dotted green arrows. I am touching the batten.
Another way to generate force along the upper surface is to fabricate ‘H’ clamps of angle iron. Clamps like this work well but are expensive to fabricate.