finally got my granddads post vise mounted so I can use it.  It is a no name 5 inch that is in ok shape.  Thanks dad!   

so the mount is mounted to a plate, that is then screwed into the sides of the post? is this because the top of the post wasn't strong enough for lags? or because the lags being used you didn't think would be long enough? or maybe just because you wanted something solid you could beat on if you needed to?

Looks like it'll work, but it's missing it's spring.

Agree with neg looks like a great vise but just needs the spring to open the jaws.

so the mount is mounted to a plate, that is then screwed into the sides of the post? is this because the top of the post wasn't strong enough for lags? or because the lags being used you didn't think would be long enough? or maybe just because you wanted something solid you could beat on if you needed to?

Screws, (or nails), don't hold well in end grain.

How much lifting force are you expecting to apply to the vise mount?

Since your spring is missing, you can make one out of mild steel bar stock (or spring stock if you prefer) easily enough.  Just find some close-up side pictures of other vises for the geometry.  Make your spring and put that nice looking vise to work!