There are still a number of factors that come into play regarding the vests that you mention above. In the early days of soft concealable armor, most vests were made of 100% Dupont Kevlar. Since then, vests are made of a variety of different materials to include Kevlar, Twaron, Artec, Spectra, Spectra Shield, Dyneema, Felt Core, Gold Flex, Goldshield, etc. More often than not, the vest is made out of a hybrid of the above materials.
Under the old standard (NIJ .05) our company manufactured several different IIIA and two different level II packages that reliably defeated the 5.7 rounds in the SS195 and SS197 configurations. We also defeated these rounds against a level II package that is NIJ .06 certified.
It was my experience with both the previous standard and the new standard, that armor made with a woven material such as Kevlar, Twaron or Artec on the threatside, and then backed with unidirectional laminated material such as dyneema on the bodyside to reduce trauma, perform the best against the 5.7x28 threat.
When shopping for armor ask to see a list of "Special Threat Testing" rounds. There are both NIJ .05 and .06 certified vests that will defeat these rounds and there are .05 and .06 vests that will not successfully defeat these rounds.
Reference to the vest that you shot and had the penetration, find out what materials were in the armor, I am thinking largely goldflex or something similar. If it was constructed of kevlar or twaron other factors come into play such as the particular denier (threat thickness) the material. There are several different deniers. The age of the vest and the manner for which it was cared is a variable. I always remind people, treat your armor the same way you treat your duty pistol/rifle. It is a life-saving piece of equipment.
Also, you say that the vest was "draped" over the clay. When we shoot against clay the vest is strapped against the box as it would be a human torso and yes, this can make a performance difference.
Attached is a link to the NIJ site that lists the manufacturers that have armor certified under the new NIJ 0101.06 standard.
http://www.justnet.org/Pages/06_CPL.aspx
Sorry for the extended post, just wanting to help where I can.