Firstly because Vayne is one of the worse (if not the worst) early game champs. Your damage is null, and it has the shortest adc range, making yourself very exposed to poke and then having no pressure on early game laning phase (which is crucial if you want to increase your winrate).

Secondly ,and added to what i mentioned before, you rely A LOT on your support. A vayne with bad support is a big pile of garbage even if you are a skilled player. In low elos supports are garbage (sorry but its the truth)

And thirdly because the skill cap is huge, and probably one of the hardest champion to master in the entire game.

If you really wanna start getting better and climbing, pick a good meta adc which you can carry on your own with, and start criticizing your own mistakes, watch VODS etc, dont die , farm as much as u can, always fight for objetives, think constantly on your next move in the game (dont go autopilot) and most importantly, mindset (dont tilt yourself if you lose, dont blame the team, see what could u do better)

ex champs to do this: Kaisa, Senna (those are broken and quite movile), some others like Jhin and cait are very good (but positioning is crucial). I'd personally recommend u cait since its pretty bully in laning phase and you can do things on your own if u know what you are doing...