I would use a bronze option. I would also heat the whole vice up prior to doing any welding, then heat it again post welding, and slow cool it.

At work, cast parts are slow cooled in a big drum filled with vermiculite. Damn parts will still be quite warm the next morning in that stuff as it is quite the insulator.

Would it be better / easier to take the vise apart and drill and tap a few oles across the back (near where the feeler gauges are) and bolt a plate on? This looks like it woudl take care of the "pads" and also add a bit of support for the crack.