There is very little public research about how well affordable (surplus) body armor really works. So I spent 50 Euros for a used ex-German police vest. It matches the IIIA protection class (protects against handguns, but not rifles, and protects against knife stabbing too).
Then I put it around a tree trunk and tested several practical weapons against it. Slingshots, a katana, my famous Zombie hammer, even a compound bow (converted into a homemade crossbow). I was very surprised -none of these weapons could pierce the fairly light vest (2kg). The only thing that did work was my throwing knife shooter.
Of course newer vests may work a lot better (this one is based on Kevlar inlays), but let's be realistic: Who can afford 500 dollar gear as an item of SHTF prepardness. These surplus products are affordable, and in plentiful supply. I can recommend them, clearly.