You could always specialize and keep an eye out for someone making a .17 HMR pistol in a concealed carry format - that would sure got through a vest. Although I wouldnt count on it taking the person out.
What about .22 (or .22WMR)? I thought I remember reading somewhere that because of their small diameter they worked well against the 'weave' of the kevlar vest. Or was that just hooey? Now I also wouldnt trust a semi-auto .22 with my life because of their rim, and a pocket pistol with 5-6 rounds in .22 wouldnt make me feel too protected.