Vayne is a micro chasm of irritating design elements. She is:
- Very safe (Condemn, Tumble, Stealth, always builds Bork which she synergizes with)
- Duels extremely well, making it hard to stomp her
- Scales incredibly hard, so you are obligated to win lane just to go even.
- Was made back in a time when "Crush them pre-6" was considered legitimate counterplay (Lol).
- Has a toxic playerbase that tells you that you were outplayed.
- Scales off of your defensive stats, making it hard to counter-build her.
These all synergize ironically. She scales incredibly hard and it's hard to counterbuild her, so she will inevitably be a monster provided she gets the right items. You then must stop her from getting them. However, everything in her kit works to try and prevent you from stopping her, due to her insane safety and duel potential. Screw up once, and a good Vayne will punish you unfathomably hard. Vayne screws up? As long as she plays it safe, she'll scale it off.
That's the crux of it, in my opinion. Nobody likes a champion that is extremely annoying, very strong and insanely difficult to punish. As soon as she's picked, you have to play HER game. The VAYNE game. Keep her behind then win before she scales back.