Usually, fragmentation is undesirable. It's better for the bullet to expand while retaining the highest percentage of its original weight as possible to increase penetration.
It's true that bullet construction varies and some bullets are built for rapid expansion (varmint bullets) and some for less expansion but deeper penetration. Velocity has quite a bit to do with this aspect.
However, the make-up of these vests and their ratings consider both penetration and blunt trauma. If you research it you'll find that some vests pass against penetration but fail to meet blunt trauma standards.
What Laz is trying to tell you is that wearing a vest does not necessarily guarantee you the "least amount of damage" or that you will live through the experience.
To the extreme,
Case 1. You're shot with say a .308 with no vest and the bullet passes through without expanding (making a .308 hole ) or hitting a vital organ and and you live.
Case 2. You're shot with a .308 with a vest and the vest slows and expands the projectile, you die because the expanded bullet damages a vital organ.
What's the least amount of damage?