Okay so me and my friends were discussing how 
could be balanced and we came up with a simple solution:
Her W is a toggle and costs mana. Something like Jinx's Q. So when toggled on, her aa could mark the enemy but cost like 5 mana per mark. But when toggled off, her aa won't mark the unit.
Her mana is too low for that. If that were to happen, the would need to cut the mana cost of her E, because in early levels it uses nearly a 3rd of her mana and is on a 20 second cooldown.
If you truly want to "balance" her, something has to give. Weaken her in one spot, buff her in another.
You do know vayne goes oom after 3 tumbles and using the rest of her kit right?
