You didn’t buy that vise, you stole it. Congratulations an a wonderful find.
If the dog moves then push it up and if there is space under it and you can place anything under it to hold it up while you remove the screw with vice grips or something similar. You could also grind a slot in the screw with a rotary tool if you have one. Application of a rust buster type liquid or heat might help break any rust bonds and help to ease the screw out which would lessen the chance of breaking it off.