She'll be pick or ban. Watch.
You know, I swear I've been been seeing people post this about Vayne every time anything involving her has happened throughout the last year. And every time, they've been wrong.
People already tried a Youmu's into Triforce build earlier in the season. It was popular in Korea for a while following the triforce changes. People eventually concluded that it was weaker than her main build, which was already weak, and it got dropped pretty quickly. These changes make that build a bit better, to the point where it's clearly competing with her main build, but it's still not enough to make her even an average ADC in terms of power. Given how completely absent she's been from pro play this season compared to other AD carries, I am extremely skeptical of the claim that a small buff will take a build people already tried, and make it pick/ban. I just don't buy it.
tumble alone deals just as much damage as her W now that it can crit
You have a misunderstanding of this change. The exact text of the change was "Fixed a bug where Tumble's bonus damage was using an independent crit roll instead of being tied to the same crit roll as Vayne's basic attack. " This means that Vayne's tumble could already crit, but the crit for the bonus damage would proc independently of the crit for her normal autoattack damage.
The funny thing is, this buggy interaction was actually good for Vayne. Bugfixing it actually decreased her expected dps in any scenario where she has crit but has less than 65% crit. The mathematical explanation for why is detailed in this reddit thread. This is why you can see a decrease in her winrate in patch 6.15 and 6.16 (the bugfix was in 6.15), despite a negligible increase in her playrate and then a decrease in her playrate compared to 6.14, and despite her actually getting a straight buff to tower damage in the same patch. This is also a factor in the switch to the Ghostblade Triforce build. Because it doesn't have crit until the third item, it's not affected by the nerf to her dps when she has crit, so it did not lose as much power as the mainstream build did.
This is a large factor in why she's not too strong right now. The recent tumble buff brought her winrate back up to around 48%, which is where it was before her accidental nerf. She wasn't pick/ban before, and going back to a previously tested build won't do it for her now.