This is what a representative of the defibrillator company said: "I am writing back to address your questions about the life vest and fire arms. First off I need to let you know we have not tested this specific circumstance. In additionally we do not have the information as to whether 150 joules, which is what the treatment of the life vest is, would be sufficient to cause any reaction to the ammunition. Possibly manufacturer of the ammunition may know what tolerances their product has."
As for security of the firearm should I be in a situation in which the vest is activated... I would think anyone who takes hold of it while the vest is discharging would probably wish they'd listened to the alarm that says, "Don't touch." More importantly -- as callus as it may sound, if the vest didn't restart my heart, I think weapon security would be the least of my worries.