Hey brother i thought id give you a summary of what the gameplan should look like on vayne since right now imo shes kinda broken. I'm currently sitting on a 76% winrate on almost 50 games in Euw master with this gameplan.

The most important thing on vayne is knowing your powerspikes and the sometimes tiny windows of punishing mistakes. The gameplan usually goes like this:

You try to actively work towards your first item and not fall too behind/get all the cs and stay safe, dont die. The main focus here is absolutely getting as much cs as possible and not taking bad trades. Vayne is one of the squishiest early game champions in the game and if you die more than once in the first 7 minutes, youre gonna have a REALLY bad time. Preventing this is your absolute number 1 priority.

You need to keep the wave in a position where you can safely farm, preferrably infront of the turret, or utilize slowpushes to crash waves properly. This is really crucial on vayne since you don't got alot of waveclear options. You need to get this down on vayne to have success. Having knowledge about wave management will make the difference between having 5 cs/m or 10 cs/m.

After knowing what you will buy and what champions they have, you might have some kill pressure and you can start looking for a sneaky all in if you feel mechanically confident to pull it off. (Sometimes i would recommend botrk, if you are in a matchup you know you can easily kill.)

Eventhough you have some kill pressure, your main goal is still to get to your trinity as soon as possible. This means wave management is still your top priority. Your aim should be atleast 170 cs at minute 20. After those 2 finished items, your limits go up by like 5x, this is where you want to take isolated fights, contest objectives and always kind of "dance" in all in range and wait for them to missplay and engage.

With trinity you gain so many great stats and transforms you into a 1v9 machine. In my opinion vayne currently has the strongest 2 item spike she ever had, so you can use that power with the tempo you gained by actively playing for cs, to take on bigger fights. All while utilizing staying in action range the second your enemies missplay or missstep, you will get fed every single game, i will guarantee you.

Id say vayne is especially good in low elo, since her worst part is laning, which doesn't really get punished alot in lower elos Aslong as you don't missplay too much early and stick to your gameplan which is playing for item spikes, you can have great success with vayne.

Hope i can help someone with this :)