I guess the "new" vest people are talking about are what we call the IBAS. The flak jackets are the older ones that most people may remember from Desert Storm.
The IBAS and the flak jackets do the same thing, they are only designed to stop fragments, and up to 9mm rounds (so they say). In the basic form the only difference is the IBAS is newer, lighter, and more sexy in appearance. How they begin to differ is that the IBAS has plates you can put into pockets in the front and back that stop 7. 62mm rounds (up to something like 2700 fps). When we first got here I did have an IBAS but no plates, we found some Haji plates in an Iraqi warehouse and used them because there weren't enough of the good plates for everybody. The Reserves, National Guard and alot of Active Duty did not even have the IBAS, but they did have the flak jackets. Supply was not meeting demand. Only meant to be for the forward units, the IBAS went to the war fighters and their support elements first. As things evolved and the wounded were more supply joes and janes and not just the infantry, the IBAS finally got handed out to everybody with plates. I just got my plates a couple of months ago. Pretty much everybody has the IBAS now, even the National Guard guys.
From a medical standpoint, both vest do their job in protecting a soldier's torso from fragment wounds. I have treated soldiers that were wearing each vest, however, just because they are wearing them doesn't protect them from injury. Especially when 155mm rounds are daisy-chained. Soldiers are more at risk from RPG's and IED's more then anything else out here, and our equipment does work, even though it may not be as cool looking as somebody else's.
Sorry to be long winded, just my observations from my last 9 months in Iraq. Hope this helps.
Scott