Just putting the build up for a friend right now, while I work on the actual guide section. =)

Vayne, the bane of tanky DPS. Or at least alleged, anyway.

In my opinion, to start with, this champion is completely misunderstood by the majority that pick her up to play. To start with, she's not overpowered, she's not the Juggernaut, she's not an initiator, and last but not least, she's not a bad champion.

It's simply a matter of understanding what you're dealing with, which is why you're probably here.

First off, let's go over her skills, the usages, and where they come into play!

Tumble (Q)

Stat-wise, this is your single-shot auto attack damage output steroid. Too many words? This makes one attack beefier than normal. Utility-wise, this is your escape, your distance closer, your skillshot-evasion panic button, or your emergency stealth switch. Keep in mind, this does not go through walls. Additionally, to use it again, you either have to wait for the skill to end, or you need to shoot someone or something to refresh it. Keep that in mind when using it to escape.

- Escaping
Bushes are your friend. Run in, juke back and combo your chaser with an E, either to knock them the opposite direction, or if possible, to stun them.

- Closing the Distance
Please don't overestimate yourself. If you're chasing someone down and you're sure you're safe, use Q and finish them. Otherwise, don't overextend yourself with it. When you use it, you're removing your ability to use it again until cooldown's gone, and remember, you're very squishy.

- Stealth
Use this when you're being focused in a fight. It's great for losing the enemy team, while your allies counter CC. Additionally, when going for ganks, it's handy when used to roll up behind the victim, in conjunction with a well-angled E.

Last but not least, in a few situations, I've used it to position myself in the middle of teamfight-induced chaos, for the element of surprise. It's not recommended unless you know exactly what you're doing, and even then, it's a **** shoot.

Silver Bolts (W)
This skill is entirely passive. I normally take this at level three, then depending on if I'm in a side lane, or in mid, I'll either leave this to finish last, or level it up secondary to Q.

Condemn (E)
This skill knocks the targeted champion back, and if said champion collides with terrain, it will stun them. Pretty nifty. When ganking, come up behind whoever your victim is, and knock them towards your ally. If you're trying to escape, it's a great mechanism when used in conjunction with Q, as mentioned above.

Please don't try to use it as a finishing move, unless you're able to stun them with it. Failure to do so will result in them escaping, and you looking like a dork.

Final Hour (R)
First off, that is an absolutely kickass name.

This should be on during pretty much every team fight. It should also be on when you're going in for a gank, given it applies a short stealth duration when used with your Q (Tumble), and additional movement speed when chasing your enemy down. The cooldown is roughly a minute long.

Marks

x9 Armor Pen.

This gives Q the ability to pack quite the punch at early levels. Additionally, it's an overall good choice on just about any AD carry out there.

Seals

x9 Armor

Survivability is horribly lacking on Vayne. Compensating for it, means you have the ability to harass without getting your head blown off in return.

Glyphs

x9 Magic Resist

Same as above.

Quintessences

x2 Health

Same as above.

x1 Movement Speed

This is useful on almost any champion. On Vayne, it provides a little more mobility for harassing, as well as the ability to effectively keep out of range when they come to return fire.

If I'm taking mid, I start with Doran's Blade.

If I'm in a side lane, I take boots and five health pots, and plan to start ganking mid between levels four and six.

If I'm taking a solo lane, I go with Doran's Blade for the added health and life steal.

That being said, my build order is normally as follows.

Doran's Blade > Boots 1 > Sapphire Crystal > Sheen > Berserker Greaves > Doran's Blade > Phage > Doran's Blade > Trinity Force > Cloak and Dagger > (sell a blade) > Vampiric Scepter > (sell a blade) > Infinity Edge > Bloodthirster > (sell a blade) > Banshee's Veil > (sell Cloak and Dagger) > Phantom Dancer.

Banshee's makes the Tenacity factor on Cloak and Dagger a little less necessary, which is why it's sold for Phantom Dancer.

Tumble is awesome. Like, super awesome. Prioritize this, with sole exception to leveling your ultimate.

Silver Bolts does a little bit of true damage every third attack, that's fairly menial at most, when the numbers are actually written out. Take it at level three. If you're in mid, prioritize this second to your Tumble skill. Given you only have one opponent, you'll have a ridiculously easy time getting the proc off. If you're not in mid, I wouldn't worry too much about it. The only time I go about it differently, is when the team facing us is particularly beefy.

Condemn is something I like to prioritize when I'm in a side lane, given it's easier to stun your opponent there. If you're bad with it, then you'll probably want to ignore it and work on your W until you feel more confident in your abilities.