A hole is broken? Whazzat mean? Holes don't break. Holes are the absence of stuff. Is the casting where the hole is located cracked? Is there a crack that passes through the hole causing the hole to be larger than it ought to be?
Cast iron is hinky stuff to weld. It can be done but ya gotta heat the whole casting up to a very high temperature so that the welded area doesn't just break away during cooling.
I have not yet seen the vise, but I think this is what is described as the problem area. True?
I'm confused. You don't know what is wrong with this thing because you haven't seen it and the seller's representations are so inarticulate and unhelpful that you can't reliably say what is wrong?
Maybe you should not spend the $$.
If the shaft is sheared off then it can be welded back on but the cost you'll pay for such a weld is likely to be more than the value of the item.
You could also tap and drill the shaft to simply bolt a new stub end on, but only after you toss it in a lathe and turn the end
You can also just tap and drill the end for a 1/2-13 or 3/8-16 bolt and use that with a washer and some locktite to do what the pin was doing quite reliably. You'd only need to file or grind a flat spot to center the drill to do this.
Short from long: If the lead-screw is broke and you can't replace it then the vice is also broke and you shouldn't buy it.