Why do vice jaws need to be so thick?

[Frank Drew]

I do like Frank's idea about using something like leather to increase friction. Would the leather be glued on? And, I assume, on both faces.

John, yes, line both faces of your vise. The lining can both cushion your work and add some friction to the vise's pressure. Some like to use cork pads, others try rubber sheets, some of us like leather --I tried both smooth side out and rough side out and much prefer the latter, for the increased grip. I applied it with a spray adhesive (3M 77, maybe?)

An important point to remember is that no matter how big and strong you make your vise, the interface with your work should be softer than the workpiece; if you crank the bejesus out of your unlined hard maple vise jaws onto, say, a mahogany table leg, you stand a good chance of denting the mahogany.