I am AWARE that this would be OP as hell...but 
stacks teemo's poison, twitch's poison, Gnar's hyper, kali's spears...but not Vayne's silver bullets...did they have to code it to specifically NOT to apply them or is there something different about the silver bullets than the other abilites?
Randomonium2 votesVayne can only stack her silver bolts on one target, unlike any of the other abilities you listed. If you get 2 stacks on one target and then switch to another target you lose the 2 stacks on the first target. That's why it doesn't work with hurricane.
Void Nargacuga1 votesIt might be due to the wording of 
's Silver Bolts passive:
Silver Bolts http://vignette3.wikia.nocookie.net/leagueoflegends/images/1/12/Silver_Bolts.png/revision/latest?cb=20130929123423
Passive: Vayne's basic attacks and abilities mark her target with Silver Bolts for 3 seconds, stacking up to 3 times. Attacking a new enemy cleanses the previous target of all Silver Bolts.
The third stack consumes all Silver Bolts on the target to deal them true damage. The damage has a minimum damage threshold, and is capped at 200 against monsters.
JustSir1 votesso, im guessing, even thought the hurricane fires at the same time, the game doesnt register as a "simultaneous" attack?
Void Nargacuga1 votesProbably? I don't know. It does specifically say that it has to be 's basic attacks or abilities. So that could be part of it.
Papa Slothy1 votesI can only imagine the silver bolts
