How parallel are the “built in part rest surfaces”?

We machine the jaws to within .002” of parallel, to the bottom of the jaw. They will, normally, not need to be even this close as you will, normally, be removing the gripped “carrier” material anyway. If you need them more parallel for a special application, this can be achieved by removing the grips, stops and their screws from the jaws, tightening the vise with the jaws installed and machining a small amount of material from the top of the jaws. Or you can match grind the tops to the bottom on a surface grinder.