Not scientific but close enough.
Brenneke slugs are swaged lead and are softer than hard cast, let alone heat treated hard cast like Dixie Slugs.The Brenneke slug may not penetrate but will at least incapcitate and possibly kill the BG/victim. The flex in the vest will extend some inches - 4" ???- what ever tissue behind that will get damaged. Read a story about a SWAT officer hit by a Brenneke Slug and died as a bone (rib or strenum or collar bone) was broken and penetrated his heart. The Box-O-Truth tested a soft Foster style slug and still left quite an impression behind the vest. The Brenneke hard cast slug would not flatten out and would "penetrate" - i.e. push into the victim much further. For sure the bg/victim would be incapacitated even if they lived which they probably would.