I remember if 
proc'd a vital, then it would deal damage and disappear for 15 seconds. Recently they always appear instantly after she proc them. Riot hasn't mentioned anything about this being a bug, so did they just make a broken champion even stronger?
No, vitals that pop or fade both cause Fiora to get a new vital immediately, according to wiki..
That was probably an error you made along skimming the lines. The vitals appear that long, but don't disappear that long. They are gone for about half a second, and is there to simulate the feeling of dueling someone and beating them with pure skill by constantly attacking their weak points.
