Hello Tom!
A "hard cast lead bullet" will not defeat a bulletproof vest!
There are two ways to make a "hard cast" bullet, either by increasing the amount of antimony in the lead blend, or "cold quenching" the bullet in ice cold water as you drop the bullet from the mould. When you alter the antimony level in the lead blend, you alter the bullet's ability to deform. When you "cold quench" the bullet, you increase the bullet's "brittleness", and increase the likelihood of the bullet "shattering" on impact.
Lead bullets (both cast and swaged) have been the age old universal projectile, especially for gallery-type where shooting is conducted "indoors", and the ranges use some form of armor plate as the back stops!
Additionally, virtually any experienced shooter/reloader can tell you, that IFF (if and only if) a "hard cast bullet" COULD penetrate any form of bulletproof vest, it would be included along side armor piercing rounds as "banned cop killer" ammunition!
Good luck and good shooting!