So I finally have an update on this project (I hate when people don't follow up!!!). Since I wasn't going to be able to cut 3tpi threads I went with the second option after confirming the damaged portion is forward of the nut when the vise is closed completely (in other words, completely irrelevant).
I center drilled and then drilled pilot holes in each half, sized to fit pin stock I had on hand. Then I joined the two halves and put them in a machine vise using a small V block to align them and TIG tack welded the halves together. I then mounted the screw in the lathe and used a cutoff tool to make a deep groove (two cuts next to one another) next to the crack so I could get down to good metal....went at least half way to the center. I put the assembly back in the vise and TIG welded it all the way around slightly proud of the threads, put it back in the lathe and turned it flush to the top of the threads.
In the picture showing the holes in the ends you can get an idea how clean metal needs to be for TIG welding...shiny, spotless clean. The bonus is no smoke!
I forgot to take a picture of the groove I cut after tacking the two halves together...you can get an idea how wide it was from the area I filled with weld. I overlapped the weld from the deep groove into the area where I had the tack welds by cranking up the amperage and burning it in deep.
I then replaced the bent original handle with one I made out of 1144 and turned to fit the spindle nicely. The ends of the handle are turned down for the ball ends to slide over, and the ball ends are swaged in place using an oxy-acetylene torch and a big hammer. I actually went back and blended the end of the pictured ball end a bit better...I like it when you can barely seen the line.
I have no idea how long it will last, but at the very least my buddy will have his vise back in operation. I'm going to tell him no more cheater pipes on the handle!
