This is what happens when you quietly and subtly buff a champ for months.
Since her last nerf, she's gotten buffs to her itemization (5% crit removed from Infinity Edge and moved to Phantom Dancer, she builds IE 3rd item at the earliest, and sometimes not at all, while Phantom Dancer is core on her), a bugfix where she got extra AD on her ult tumbles removed, but given extra AD on her ult which is a net buff, buffs to Blade of the Ruined King, a favorable meta change).
Yes, these are small quality of life buffs and bugfixes - but they're buffs nonetheless.
Vayne's power right now is just a perfect storm of several small factors that have pushed her over the top.
Wanna know what meta would destroy Vayne? Watch S4 worlds again. 2-3 assassins per team, Lee Sin everywhere who's way more mobile than she is, Lucian/Trist who both stomped her in lane, supports more about making picks than peeling.