For a moment, lets forget that Tumble provides any kind of damage and just think of its mobility.
The DPS increase from this buff might be a problem but it's not the problem I'm talking about here.
It's a 300 range dash with a 2 second CD after you proc its attack and, if your ult is active, it provides a 1 second stealth on cast.
A 1 second stealth will hide the direction Vayne tumbles in as well as where she walks for the remaining ~0.8 seconds.
This means that when she presses Q during her ult with the intent to juke your ass, she could "reappear" anywhere within a ~600 radius of where she pressed Q.
If she has to wait 2 seconds after getting back in range of somebody before she can stealth again, I can justify this power.
It's a great playmaking tool, but that 2 window is enough of an opportunity for a team to react, pounce on her, scream her name a few times, and kill her.
A 1 second window, however, is not enough for me to justify that level of power.
If you go through with it, the stealth should do little more than hide the direction she tumbled in.
Reducing the stealth duration from 1.0 > 0.6 would be enough.
Honestly I feel this is the wrong way to buff Vayne.
If you want her to be stronger, give her ult some of its AD back (or, because you're Riot and hate going back on a decision even if you really want to, give it an AS steroid).
That said, if you're going to do this you should at least get it right the first time around instead of leaving it broken for two patches before nerfing the shit out of Vayne.
Also, I know Vayne doesn't really buy any CDR, but we all know somebody in our solo queue games is gonna go tank Vayne top with Triforce/Frozen Heart solely to experience 0.55 second CD's on Tumble. This is a reality I want to avoid.