it is a combination of the velocity and the shape of the bullet piercing the vest. The same thing works for rifle rounds. Note that the round is concave instead of convex to the point, which makes the point have a lot more pressure on it.
And, just so you know, the II-A vest is not rated to stop even a regular 9mm at those velocities. II-A is tested with 124 grain round nose FMJ (military ball ammo) at 1225 FPS. Threat level II armor is only tested at 1305 FPS. The closest I could see as a standard test is 357 SIG Flat Nose 125 grain at 1470 FPS. This is the standard for a level III-A vest.
for some interesting reading on testing and the vest standards, the newest standard is now available at
http://www.ncjrs.gov/pdffiles1/nij/223054.pdf.