In my humble opinion, a level I vest is adequate for civilian defense purposes. The rule of thumb is you should wear protection adequate to stop the caliber of the weapon you are armed with. If you carry heavier than .380, you may want to Step up to II or IIa protection. However, at that point I would rethink my risk profile and comfort level therein.
If your risk profile indicates Level II protection, than a high capacity 9mm handgun with at least one spare mag would be a reasonable expectation to accompany your load out. This all leads to encumberance and concealability issues. I personally would not go through the discomfort of wearing this much gear on a daily basis. After all, the majority of firearm fatalities in the United States are inflicted by small caliber rounds, and Level I protection can save your life in these cases.