The Prentiss vise can readily be repaired by brazing. Simply clean the casting meticulously, removing every trace of dirt/oil/grease, aka 'congealed crud', and 'V' out some distance beyond the visible crack (drill a little 'crack-stopper' hole a bit past the end of the crack).
Pre-heat the entire casting to a 'black heat', then braze up the crack normally, building up a little to allow for milling the casting nicely flat after it cools. The sides of the beam may have spread slightly, and may need to be filed/scraped back to parallelism, just enough to slide through the fixed jaw smoothly.
I prefer a 'nickel-bronze' brazing rod for this work. Correct pre-heating of the entire casting and careful slow cooling is absolutely essential for a proper job of this repair.
(This crack is surprisingly common on older Prentiss vises. Many of the Prentiss beams have a weak spot there, suspected to be caused by a 'cold shut' in the original casting process. Once correctly repaired, the vise will be 'good as new', and will serve 'indefinitely' long if not abused.)
cheers
Carla