Apparently, the passive doesn't apply. At all.
I'm playing a custom game right now and I'm testing it.
With or without vital, the damage is the same.
It does not consume the Vital when you hit with auto-attack OR lunge.
It does not heal you.
It does not do true damage.
It does not give move speed.
The whole passive is non-operational.
EDIT - Will add more bugs or information as it comes in from my own experience or others who post here.
Serika Zero says:
Q doesn't always go for Vital
Has played against Fiora and the passive worked
(maybe the bug is isolated to custom mode? will test in blind pick and add info afterwards)
EDIT 2 - tested in normal game, still doesn't work
EDIT 3 - apparently, it works, but Riot forgot to mention that "when you hit a vital" means you need to hit the side of the champion the icon is showing
Will leave the thread up and further investigate. Also anyone can read this and know about it now.
EDIT 4 - works with ult and passive well when hit from the side of the champion it's pointing, no issues so far
tbh if you're gonna make a mechanic like this you'd also wanna write down what you're supposed to do to trigger it
EDIT 5 - the ultimate has a prime time, it will not detect procs on vital spots in the first, 0.5-0.75 seconds of casting it, this shouldn't happen as far as I know
Several games later, definitely confirming it. The passive doesn't proc in the short time after casting ultimate. It works fine after that, but if you for example, use R and then Q, it will not proc.
EDIT 6 - the ultimate vitals do not proc if you SOMEHOW manage, which I did, to hit between them
aka, you will not proc any vitals if you hit at a 45 degree angle from any horizontal or vertical line drawn from the center of the champ