The vayne buffs targeted the wrong thing.
Riot buffed vayne because she needed to be buffed, but they buffed the wrong things. Instead of buffing her W, the core part of her kit that offers the most reactionary counterplay, they buffed her Q and gave her insane burst with the critical strike build. She definitely needed some buffs, 46% winrate, low winrate among her mains, no LCS play, she wasn't doing well but buffing her burst damage was a little dumb. EDIT: an alt way of buffing her could have been to decrease E CD or decrease Q cd (but keep Q damage the same or slightly less)