I'm Actually a Fan of the Vayne PBE Changes. (Edit: PBE Changes gone. Thanks to Archerno1 for info!)

Edit: Apparently the PBE changes are gone. That's a shame, they were a right step towards making Vayne less of a ranged Assassin and more of a ranged DPS hypercarry. Thanks to Archerno1 for informing me.

I already see the preemptive comments and downvotes incoming for saying that since Vayne is one of the most loathed champions on the Boards, but just hear me out.

Vayne in her current state is not Vayne. Vayne is meant to be an extended-fight skirmisher that specializes in busting tanks and bruisers while having enough maneuverability to play keep-away from the enemy ADC. However, Vayne right now is really just a ranged Assassin with an absurd 2-item power spike and no fall-off timer. Her build of Shiv > IE might not seem problematic until you realize she is able to start 3-shotting most champions with just 2 items. This is not healthy or fun.

The PBE changes have moved a considerable amount of power around and aim to fix this issue.

Let's get this out of the way since people have been screeching about it: Vayne is not receiving a net buff. This is still a "Buff", but it's hobbling an area she was not intended to excel in (her harsh early-mid power spike on 2 items) and putting more emphasis on where she was intended to excel. (Strong scalings and hypercarry potential at the cost of an easily-abused early game.) She is a skirmisher that likes to fight in longer 10-second duels, not an Assassin meant to kill her target in one shot. So please just stop crying bloody murder that Vayne is receiving buffs to 2 of her abilities while overlooking that another is getting nerfed badly. Calm down, apply some thinking, and follow through with me on this. OK? Good. Now let's begin the breakdown.

1. Her Q bonus damage not being able to crit anymore is the perfect nerf to Tumble. Being able to R-Q-Auto out of a bush and instantly kill the enemy ADC is not Vayne's playstyle. Now that Shiv + IE won't be nearly as good with this change, I think we'll start to see much less Assassin Vayne and more Skirmisher Vayne. In fact, she might be able to start using Fervor to its fullest effect since she won't be looking to instantly kill her targets. Whether Fervor will be revived or not remains to be seen, but I'm confident that this Q change alone will shift Fervor in Vayne's favor.

2. Her Silver Bolts compensation buff is also a very smart change. Buffing this to scale better with each rank and have more per-trigger value will give incentive to focus more on her sustained damage instead of going for that OHK. It also puts more emphasis on her tank-buster identity, which is kind of why she's an iconic ADC to begin with. Whenever I think of ADCs, Vayne always springs to mind immediately and her ability to shred tanks always makes her stand out to me. Any ADC can just sit in the back and take potshots at the enemy ADC, trying to get their crit to trigger first. Few ADCs stand toe-to-toe and duel their enemies on the spot, using superior mechanics and positioning to shred their opponent. Silver Bolts being buffed will encourage Vayne to use her mechanics to the fullest potential, looking to maximize the number of times they can proc Silver Bolts reliably rather than just R-Q-Auto out of a bush with IE-Shiv-PD like an actually-viable Evelynn.

3. Her Final Hour buff is also a smart change. With this change, Vayne is rewarded for staying in longer fights and busting down tanks instead of going for OHKs on enemy ADCs. I think it will need close attention to its numbers to not get out of hand, but I think this is a very positive change that shifts Vayne's power more towards where it needs to be: longer, more slug-fest engages. I find slugfests extremely entertaining in League, even if that's a controversial opinion. Naturally, ones where there is just no end and draws games out to 45 minutes regularly aren't very fun (see Season 5 for that.), but proper ones are really engaging.

4. I firmly believe this will increase Vayne's skill expression. With the power shifted to longer skirmishes, positioning and mechanics are going to be extremely important to mastering Vayne, moreso than now anyways. Currently, Assassin Vayne has a bit too much margin for error since getting close isn't that big of a deal. She's still going to 3-tap you with her absurd crit damage. However, with these changes, she's not going to have that error since she won't be able to 3-shot you as efficiently anymore. She needs to stay back, time her ult, use her E well to self-peel in danger, and save Tumble for when she's about to get dunked.

So yeah, I'm a fan of the Vayne changes and hope they make it through to shift Vayne's power around where it belongs. Maybe it won't kill Assassin Vayne, but it's the right steps to start chipping away at it and bringing back a more slug-fight Vayne. Maybe BoRK + LDR will start getting built more often too. Who knows.