As Vayne you should primarily focus on farming in the lane phase. Vayne has a weak early game compared to almost every adc. The good thing is that she does very well in farming under tower because of her Tumble which adds extra damage

The thing that most low elo players struggle with is that they wish to not use Condemn to get that 1 minion. However, the cooldown on it is not substantially long, and most likely, you will not be needing it in the next 15+ seconds.

Pros:
- Vayne has very strong kiting potential against champs with no dashes or slows.
-Strongest ranged duelist during late game.
-Snowballs very hard if you get early kills or level advantage.
-Mana conservation is easy on Vayne, and only Condemn has a high mana cost.
-With the right mechanics, you can 1v2 the support and adc if you are not Exhausted.
-Generally a monster in lower elos if you have experience and knowledge about Vayne,
- Vayne is easier to learn than some other ADCs such as Draven.
-No skillshots, so no spells wasted.
-Has a long 1.5 second stun to single target if Condemn is used correctly.
- Vayne's ultimate Final Hour gives Vayne stealth after each Tumble.
-Great at chasing down champions with her passive.

Cons:
-There will be nothing you can really do if champions like LeBlanc,Talon,Tryndamere have snowballed out of control.
- Vayne is very reliant on the success of the team, and can only carry if other lanes are not feeding.
-By far, Vayne has the weakest early game out of all adcs.
- Exhaust will shut Vayne down very hard, but Quicksilver Sash will counter that if you did not take Cleanse.
-Your range is very low, and you do not have any skills that deal damage instantly besides Condemn.
-Since there is a slight delay on Condemn, you may save a player if they use Flash right when you Condemn.
- Condemn is a conditional stun and you will need a structure behind a player to stun them.
- Vayne gets demolished by heavy cc supports like Leona, Annie, Nautilus and Thresh.

Flash is generally a must-have for all adcs. It is pretty self-explanatory and it's purpose is not something I think I need to go discuss about in detail.
Long story short, It gives you kills or lets you out of sticky situations.
Cleanse is a very situational spell. It also requires reaction time and knowledge as to when to and when not to use it. This is a very useful spell on Vayne and I highly recommend this if you are good at adc or if you have the reaction times. This spell also helps with lane fights against champs such as Annie, Leona, and Thresh. Also, Exhaust is one of Vayne's hardest counters, and cleanse will take it off immediately. Overall, if you can survive lane and play for the late game, cleanse will be infinitely more useful in team fights when the enemy team tries to land all their hard cc on you to start a fight.

-Try not to use Vayne's Condemn as an execute unless there is a structure behind your target. You do not want to be that kid that Condemns a free kill away.

-Do not Tumble to facecheck bushes, instead if you really need to, just walk up to a bush and save Tumble as an escape.

-Do not pop your ultimate Final Hour when executing a target. Rather, just pop it when a fight is about to occur to utilize and get the most value out of it.

-Never challenge champions such as Gragas, LeBlanc, and Wukong. You will get jumped on, and your team will harass you for the rest of the game.

- Vayne should not be picked because you think she is "free elo." Just because her late game is strong, does not mean that you will dominate in the late game. When a Caitlyn is 2-3 levels ahead of you at 20 minutes, it will not matter who has the higher late game scaling.

-Remember to farm the double golems when you are on blue side. They will be a great source of exp and gold for you.

-Try to use Tumble to pop the 3rd proc of Silver Bolts. Nothing is more awkward then using it to get the 2nd proc and having to walk away because the enemy adc created too much distance.

-You can Tumble right after the projectile launches from your auto attack animation to cut it short.

-You can also Tumble into a wall or structure to cut the Tumble animation off and get a quicker enhanced auto attack.

Vayne goes well with Sona,Soraka,Nami(as her early game is her weakness, supports with heal and utility is better for her) but she can also do well with Nautilus,Thresh,Leona,Blitz ( heavy CC supports)