How to fix this bench vise that has a missing handle
Use a long "grade 5" bolt, grind the head round and ease the edges a bit so to use it will be comfortable. Set it in place and run a stop nut on the other end and grind it in the same way. Do not get it too hot from grinding, it may melt the nylon part that keeps the nut in place.

There will be issues while doing this, but to grind the bolt first before placing it in the vise will at least make one part of the job easy to do. The other part, the nut, well....

You will only need to do it once.