If you took your vice and flipped it upside down you should see something similar to what appears in the attached photo. This is a Reed 33 vice that I bought several years ago, and it was missing the swivel base. So I machined one for it out of case carburizing steel, and had it heat treated. The vise weighs about 120 lbs and has 5” wide jaws. You can see in the photo that there is a huge cast wing nut that threads onto a threaded shaft protruding out of the base of the vise. Below the nut is a large steel spacer. Below that is a large cast washer that would contact the underside of the bench where the vice would be attached. The swivel base I machined would be mounted to the top of the bench, and I machined four threaded holes in the large cast washer that line up with four holes in the swivel base. The four mounting bolts sandwich the bench between the swivel base and the large cast washer, producing a very secure mounting set up. And you can see in the photo that a semi-circular notch in the base of the vise engages a notch machined into the swivel base.
It appears that your vise has been modified. The threaded rod is missing, and two holes were drilled in the base of the vise to mount it directly to a bench. That should work fine, but your vise will have a fixed base so it can no longer swivel.
Maui