Update:
I screwed up, I literally screwed up. I ordered the wrong thread pitch! somehow I had it stuck in my head that it was a ⅞"5 tpi lead screw not sure how I came to that conclusion but it was set, a couple of years ago, when I first started this project, did I double check, no, did I even measure it, I don't remember. I just ordered it... it actually is a ⅞" 4 tip thread. Anyway, the worst thing is that thread doesn't seem to exist in the acme screw world so now I'm faced with making it from scratch, which I will, eventually.
in the meantime I painted the vise and put it back together. I figure with about 2" of thread in the nut the screw has plenty of engagement and the eroded threads shouldn't cause too much damage till I have the time to make a new screw.
I even made a new end cap from aluminum.