The main thing is to keep the squirrel wannabe mechanics from putting a piece in them and then pulling the wrong way on it with a 4' pipe wrench.  The jaws are plenty strong if you don't do stupid things....ours seem to all get torn out....about throttled someone for tearing them out of the brand new vice on the welding table. 

Weld the jaws in....if you weld them well it will hold a lot better then the little screws.