You can use two pieces of angle aluminum as jaws in your vice. Place/tighten the tip end of the screw into the vice, clamping on the unthreaded part of the screw near the tip, with the hex end sticking straight up. Place a small, strong pipe or tube down over it and lever it straight. Easy does it, don't jerk the pipe, just steady pressure. take it out and check after every little effort. Don't try to do it all at one time, you'll go too far and come back and it will snap off.
