Lead is a non-ferrous material. Magnets don’t affect the trajectory of bullets, so a ‘magnetic’ vest would be no more effective than a non-magnetic one in stopping bullets.
Grapheme is a very tough material. In sufficient quantities, it might increase the speed of sound in spider silk, helping to transmit the shock wave from a bullet across the shoot pack surface, improving its anti-ballistic performance. That would make it very stiff, of course.
I doubt there has been sufficient r and d to evaluate the practicality of this approach. If you have some references, I would appreciate a share.