Would NIJ level II vest lessen the damage of a higher round?

I would guess that a vest that allowed a bullet to penetrate would cause more damage than just the bullet. It bullet would expand outside the body or yaw and make a bigger entrance hole along with the normal or larger exit hole.

Well, a number of trials have been done. Even lighter vest may stop the bullet from going through....But, the vest is just going to get pushed right into your body and make a much bigger wound channel.

Quit a few years ago ( in the mid 70s) a police officer withing 40 miles of my little town, was shot on a traffic stop with a .444 Marlin lever rifle. Bullet never went through the vest. The vest went through the officer.

Getting into rifle rounds you pretty much need the steel/ceramic plates used by military and I haven't seen that type of vest sold in retail anywhere. Blunt trauma alone will do the job.

Rifle round will go through a level II vest, through you, out the other side, and into the guy behind you. With how far technology has come in body armor, mostly considering weight and price of IIIa or III for that matter, I don't know why anyone would still use level II

even if the vest stopped a "pass through" a person would still be at risk from severe internal bruising and bleeding from the impact trauma.

judging how a II vest 'protected' a jug of water i'm going to say no.

Nope.