Hm.... I'd say that If she was given an active on her W, it'd work similarly to Kassadin's E. So that she needs to proc her Silver Bolts so many times (each time its proc'd. she'd get a stack, which will fall off slowly while she's out of combat) and THEN she can use her W active. This way, it doesn't change her core playstyle and rewards a vayne that knows how to proc silver bolts often in a teamfight instead of always swapping targets.
For potential W actives...
Option 1)
2-3 seconds of true damage to display how fierce and determined Vayne can be as a Hunter. It can't be reached easily and is a huge payoff for a vayne that played properly. as well, it allows for counterplay by simply cc'ing vayne if she activates it. and if bad vayne's get enough stacks, it doesn't necessarily mean they'll make good use of her W active.
Option 2)
Vayne leaves a trail for 3-5 seconds behind her that buffs movement speed for her and allies on the path. This allows for an ally to quickly follow up with the vayne, especially if Vayne is chasing someone down with her passive + ult movement speed, OR allows her to make a retreat from a threat that she underestimated because she "has no time for nonsense."
I feel like she's already building enough attack speed as part of her build and too many other adcs already have AS buffs built into their kits (tristana, draven, varus, etc)
Also, hello again Caítlyn [slayer-jinx-catface]