Hey there, I'm Asdw120(I know the name is awkward to say, but bear with me please).
Welcome to my first champion build guide! I decided that since there is going to be a new Season, I might as well put in my two cents on how to build my main champion, Vayne.
Since I'm new to this, I'll be welcoming any constructive feedback, so don't be afraid to give criticism, as long as it's not the trolly, uninformative kind, such as "You must be Bronze Five, Scrub."

For my runes, I have the standard 9x Tier 3 Greater Mark of Attack Damage, 9x Tier 3 Greater Seal of Armor, and 9x Tier 3 Greater Glyph of Magic Resist. These should be self-explanatory. In regards to my quintessences, I like to run 2x Tier 3 Greater Quintessence of Attack Speed, and 1x Tier 3 Greater Quintessence of Attack Damage. Now, the only real reason I run one AD Quint is personal preference. I like having the extra damage to assist in trading pokes early game. However, if you prefer to, you can switch the AD Quint for a third Attack Speed Quint.

For Masteries, I have eighteen points in Ferocity, and twelve points in Cunning. For Ferocity, it should also be self-explanatory, however, this time I will take the time to explain my choices, as they are still relatively new.
In Ferocity, I chose to go with Fury over Sorcery because Vayne only has one damaging ability, which is her Condemn(E). This ability has a fairly long cooldown, and only having one of your three controllable abilities able to benefit from this mastery does not make it worth investing points in.
Next, I chose the Double-Edged Sword. The reason I chose this, is primarily for mid-late game, where you will be doing most of your damage. 2% more damage would most likely still end up being fatal late game, whereas the additional 2% damage will most definitely help in taking out bulky frontline tanks in a teamfight. It is debatable whether or not to chose Feast instead, as it provides an early lane sustain. However the set +20 health every twenty seconds will not help late game. 20 hp, for the most part, is insignificant compared to the amount of damage that is guaranteed to be taken during the course of the game. So the chances of this mastery saving your life after the laning phase ends is pretty close to zero.
For the third level, either option is actually good. In my case, I prefer Vampirism for the additional sustain to help my early game, especially since I do not start Doran's Blade, which has a 3% life steal. So this mastery is to make up for it. You don't have to chose this mastery if you don't start Doran's Blade, as well. The additional +10 AD at level 18 does have it's uses, especially because Vayne's W procs a percentage health true damage on the third consecutive hit. So I would recommend toying around with these two to see which one you prefer the most, and go with that one.
For the fourth level, I actually don't like either of these. Of the two, I prefer Oppressor over Bounty Hunter. This is because in teamfights and during the laning phase, you will have a partner of some form with CC of some kind(Most likely). In this case, you will get a guaranteed additional 2.5% damage buff. However, Bounty Hunter is entirely dependent on your ability in taking out your enemy team. In addition requiring you to kill every single person on their team at least once, it also caps out at 5%. This is not entirely bad, except if you happen to be losing. No matter how good a player is, there will be bad games, and Bounty Hunter will just make it a bit harder to compete in those bad games, especially if your early game gets abused a lot by your enemy laners.
Similarly to the first level, this should be self-explanatory, as it is a cut-and-dry separation between AD and AP. You want to choose the AD choice in this case, as Vayne is an AD Carry.
And finally, for the Keystone Mastery, I chose Ferver of Battle. This is debatable between this Keystone, or Warlord's Bloodlust. However, I feel that this particular Keystone is stronger on Vayne, as she revolves around auto attacking, so stacking ten stacks should be relatively easy. The only way that you will not get the benefits of this mastery, is if you don't attack anything, or if you attack once every five seconds, which seems highly unlikely, as you are, once again, an auto attack based champion. At level 18, Ferver will grant you an additional 80 AD at ten stacks. That's more damage than most AD items! Warlord's Bloodlust relies on crit chance. In addition to not being guaranteed like Ferver is, Warlord's heals you based on the damage dealt by the crit. If you hit 1000, you'd heal 150. 150 isn't a lot if it's not happening almost every hit. Otherwise, your health bar will go down faster than it will go back up. Both masteries are good, but I personally feel Ferver is the stronger choice, as it is guaranteed bonus damage.

In Cunning, I chose my first level to be Wanderer over Savagery. My reasoning for this is because it allows me to return to lane, catch up to my team faster, or help me run away easier. In my opinion, if you don't have an existing problem with collecting cs, Savagery really isn't necessary, as it only deals extra damage to minions and monsters.
For level two, I chose Secret Stash over Runic Affinity, as you are most likely not going to be jungling with Vayne, and any camps you do during the game, you should already be strong enough to take by the time you start doing them. In addition to that, Secret Stash makes your health flask last longer, and buffs your health potions if you choose to buy them. Basically the more useful of the two for laning in my eyes.
Next, in level three, I chose Merciless over Meditation, as I never had any real problems with managing Vayne's mana. And Merciless will allow me to deal more damage to injured enemy champions, making the kills more certain.
And finally, for my last level, I chose Dangerous Game over Bandit, as you should be the one collecting cs during laning phase, and creeps don't matter as much late game. In addition to that, Dangerous Game will help in the late game during teamfights, as the additional 5% health and mana will allow for snowballing potential, as the heal may help in surviving fights with multiple enemy champions early-mid game.

For your starting items, I would recommend Long Sword, as it gives you quite a bit more damage than Doran's Blade, and that I have life steal woven into my Masteries to compensate, and Refillable Potion, as it continually provides you with free potions. This is incredibly helpful in matching your opponent's early game, as you will have more powerful items, and sustain. In addition to that, always grab the free Warding Trinket at the start of the game.
The first item you want to build on Vayne would be Blade of the Ruined King (BoRK). This item grants you an active that steals 10% of your opponent's total health, and 25% of their movement speed. In addition to that, it's base bonus stats are an increase of 25 AD, 40% Attack Speed, and 10% Life Steal that applies to 6% of your opponent's current health as you hit them with auto attacks. This is quite possibly the biggest damage jump you will have during the game.
For your core build, these should be your first go-to items in every game you play as Vayne. However, not all games have the same circumstances, so do not be afraid to change them should you require a different item composition. Remember to get your Farsight Alteration for your warding trinket. It reveals an area for a limited duration, and leaves behind a visible ward that will last indefinitely until destroyed, or the game ends.
When looking at which of the conditional items you want to build, it would more or less depend on your enemy team. If the opponents start to build any form of armor that reflects damage, The Bloodthirster will help you counter that damage. If your enemy duelist is extremely powerful, Death's Dance will buy you time in engagements to either win, or survive. Rapid Firecannon helps tremendously with taking objectives, so it is useful in coordinated split-pushing, or a full-team push down a lane. Infinity Edge, and Static Shiv are both very strong items should you be extremely ahead of the enemy team, or if you just want to capitalize your damage output.
Your conditional items are items that you normally would not build on Vayne, but are still ones you want to consider. Mercurial Scimitar is handy when you run into a lot of hard cc in your enemy's kit. Maw of Malmortius comes into play when your enemy apc is powerful enough to make you worry about your survivability. The two Last Whisper variants are conditional to the number of tanks you are facing. Lord Dominik's Regards if there are plenty of tanks to kill, and Mortal Reminder for those duelists and squishies that have a lot of life steal. Banshee's Veil is helpful if the majority of your enemy team composition revolves around AP damage, and Guardian Angel is a helpful last item if you are ahead. The Vision Ward is also there in case your team is suffering in map vision, or if you just feel the need to ward up some areas that the rest of your team isn't already covering.
For your boots enchantments, if you are having to defend your base often, Homeguards allows you to be at the area you need to be at faster, whereas if you are winning, then Furor will allow you to keep up with your fast enemies. Your Elixir is self-explanatory, as it adds attack damage and extra sustain. Always remember to grab your elixir last, after you have built everything else, as you can immediately consume it if you have no open item slots.

There are several things to remember about Vayne's Kit:
-First off, your passive grants you 30 bonus movement speed when moving towards an enemy champion. This is useful when chasing enemies. Also, keep in mind Final Hour(R) triples this effect.
-Your Tumble(Q) is your primary ability that you will use. It has a multitude of uses, such as dodging skillshots, adding damage to your next auto attack, or chasing enemies. Keep in mind, Final Hour(R) grants you one second of invisibility when you cast this ability. Also keep in mind when using this to poke your enemies, that you will be open for a push as your primary dodging ability will be on cooldown.
-Your Silver Bolts(W) is a passive ability that grants you percentage true damage whenever you land three consecutive hits on the same target. Be sure to get this ability maxed out first, as it is the source of a lot of your damage, and frontline removal. Something to keep in mind, is that your Condemn(E) also applies a stack of silver bolts.
-Your Condemn(E) is your only enemy displacement and CC in your kit. If you push the enemy into any form of terrain, they will be stunned in it for a short duration. If not, then it will simply push them away from you. This ability is particularly useful when dealing with champions that have gapclosers. It will allow you to get some distance between the two of you, and give you time to activate Final Hour(R) for a retaliation. Also, keep in mind that when you have someone stunned, do not Tumble(Q) when your Final Hour(R) is active. There is no sense in going invisible to avoid the attacks of someone who can't attack back. It's best to save it for when the stun wears off.
-Finally, your Final Hour(R) grants you bonus damage, and additional effects on some of your other abilities. It is important to use this in the majority of your engagements, as it could turn the tides in your favor. Don't be afraid to end up wasting it, as it has a fairly short cooldown time, and mana cost.

For summoner spells, I recommend Heal and Flash. Heal is helpful in surviving near death situations, and for turret diving. Although it does cast a debuff on you, in which you take reduced healing for a short amount of time, it also gives you a minor speed boost. You can use this to catch running enemies, or to avoid a slaughter. If that extra speed boost allows you to escape cc that would end up killing you, don't be afraid to use it, even if you are at full health! A summoner spell for a death is a worthwhile trade.
Flash is the general summoner spell that is run on practically every champion, as it has many uses. It can help you in escaping enemies, with your positioning in a fight, with dodging skill shots and cc, and for securing a kill on an escaping enemy.