Some gameplay for Vayne

Silver Bolts gets a short internal cooldown on targets, similar to J4 passive, Naut passive, Rek knockup, etc. Proccing bolts (all 3 stacks) on a target resets the cooldown on everything else. In lane she won't really feel this, in teamfights she's basically MF with Love Tap.

I don't really feel she needs a nerf and I'd be fine with her getting a compensation buff for this, it just seems like her teamfights would be a lot of fun. You can still stealth defensively and just dps one person, but you'll get the most use out of W if you control the fight and alternate between hitting multiple targets 3 times. This could add a new dimension to using Q stealth to reposition.

Playing against her, teams are rewarded for having only one person Vayne can focus, which cuts her dps from bolts. Dog-piling her, if she manages to survive, lets her put out her full potential dps. This is different from playing against MFs Love Tap, since Vayne can actually use her tools to possibly survive the dog-pile.

This wouldn't really hurt her dueling against squishies since she won't need more than one set of bolts to kill them once she gets going. Might even buff it if she gets a W buff because of this, but I like her dueling anyway.

Visually, this could be a broken silver ring around the immune enemy that shrinks over a few seconds, indicating that bolts cannot be stacked on them.