An extremely simple yet powerful card, Bulletproof Vest remains at the pinnacle of efficient defensive options in terms of cost, value, and flexibility. As a 3xp card, this can easily be slotted into any investigator's deck without much thought:
Need a way to circumvent Azure Flame (5) from burning your Father Mateo? Take Bulletproof Vest and watch those self-destructive flames turn to ashes. Your Norman Withers took a bit too much Physical Trauma over the campaign? Slap on a Bulletproof Vest and watch his effective health nearly double in one action.
In terms of action efficiency, Bulletproof Vest is a simple 1 action, 2 resource card, with no setup needed other than simply drawing the card and having a body slot open. For Mystics, this slot competes with Robes of Endless Night (which is only for Spell-Mystics), and Astral Mirror (for Arcane-Mystics), neither of which are highly used even in their own respective archetypes. As for Seekers, their alternative tends to be Lab Coat, which is a possible +1 on skill tests and is good if your Seeker tends to be doing more skill tests that are closer to DC match.