As pointed out earlier, part of the casting of the main part (static jaw) holding the two pins that holds the main spindle screw nut has broken off/cracked preventing the vise to work.
I see the spindle nut sticking out the back but wont come out until you remove the cast iron end cap (be careful). I don't think its the end of the world but far from ideal. Maybe rotating the nut 90 degree's and then drilling new holes (top & bottom) for the pins holding the screw nut in place? Could be a fairly easy solution but Wont be original anymore but should get it back to working fairly well. The casting is a cylinder shape inside and I think the nut can be rotated inside the casting? The handle cap end has been removed or broken off, it's much less of a problem, many used vises have this issue and fixable. I can't imagine the forces someone applied to this vise to make the casting crack like that on a 800? I have seen this happen when vises have fallen off a table to the ground while the jaws are open, it always wants to fall head first, its ugly to see (trust me, RIP my Athol 6"). Or maybe they tried using it as a puller, putting undue loads on the pins and casting?
I recommend the first thing to do is remove the back end cap and then remove/unscrew the nut, then remove the spindle screw. You'll now have good access to the back of the slide where you can shoot penetrating lubricant like Kroil or Deep Creep Seafoam and let it soak for a couple of days (back end pointing upward), then heat it up to help expand it loosening things up. You could then gingerly drive the slide/dynamic jaw forward by tapping on it or using other means to moving the slide/dynamic jaw forward. Most likely the slide is bounded to the static jaw body. Please be patient, the whole process take s a while and this thing weighs an absolute ton! You almost need a hoist just to move it around or rotate it. Forklift is ideal
If you're able to do all of this, then I think your home free but more detailed photo's will be needed to better see what's going on before most here can give you good advice.
PS> Looks like there's been some welding repairs, seen in the 1st and 3rd pic?