I’m not certain, but I think something is really Fouled up here. In my world, the green arrow is pointing to the base of the vise, which was attached to your swivel base before you took it apart. The blue arrow is pointing to the nut, which should be attached inside the red piece. You pic shows it outside the base, implying the pin that should be holding it is missing.
That’s good and bad. It’s also good that magnet was not a pin through the screw, which would have been a new one to me.
With the nut outside the base, unscrewing the handle to open the jaw should leave the nut outside. Once fully separated, the nut should be pulled out the last little bit, and new pix taken.
1- looking into the cavity the nut came from, hopefully showing the groove the nut mounts in.
2- the sides of the nut, which should mate with the groove above
3- bottom of the swivel base, which should have a hole that fits into the notch on the nut to the right of the blue arrow.
