I just got to thinking... Hope you didn't throw this vise out yet... If the nut happens to be non-replaceable (I suspect that this is the case), you could remove the remnants of the old nut and insert a new one, perhaps commercially available one, perhaps a one-off machine shop job. Anyway, 'stake' the new nut in place with holes drilled through the base. There you go, problem solved. Of course, I don't know what access you have to tools/machinery, so I don't know if this will work for you or not.