|©apple from their support page|
This is how I see it going down:
"Goddammit," the Vampire curses. He's at the 24-hour Duane Reede trying to Apple Pay with his iWatch. He's all, "Again with the passcode!" and gnashes his fangs in frustration.
The iWatch, in case you don't have one, asks for your passcode when you first put it on. Apple assumes it's still you hours later as long as your heart rate is uninterrupted. For those with a regular heart beat, this probably constitutes convenient security. But it sure sucks for the undead.
Luckily most undead are immortal so an extra five minutes in a checkout line might not register on the old biological clock.