Ok, on the vanity (FWC handcannon) it has firefly. It works like normally, but if if you kill a guy with a headshot then you kill [b]another[/b] one with a headshot, the [b]first[/b] one you shot explodes [b]again[/b] (it explodes where the body is). It isn't an AMAZING hidden perk, but it does a fair amount of damage. Also it has its own special animation (a bit like a firework). Try it out! Have fun with the vanity!