First of all, this is my first Build, and more of a guide, it's an asking for your help with it, so don't be mean ;)
So, this build is based on a simple strategy that fits perfectlu SHaco, Vayne, and Nassus, and probably other heroes too, and the plus that they use the very same rune page!
SO these three champions have one thing in common, their Qspell is based on doing more damage on next attack (Nassus siphoning Strike, Vayne Tumble, and Shaco Deceive), and all three of them go well with SHeen and similar items...so, the idea is to do massive damage with their Qspells!

In the case of Nassus, this will require farming a lot his Q, so get the hang of kiling minions with it, and happy farming!
With Vayne the strategy is pretty much the same as usual, kill them before they know how you even did it.
And with Shaco the strategy is a bit more complicated, first of all kill the golem, with three or four JitB's, then go for first blood, and do what you know.

REMEMBER: This guide isn't focused on teaching how to use Nassus, Vayne, or specially, SHaco, it is focused on how to build them for massive damage.

One of the best things about this Guide is it uses one set of runes for three different champions!

Quintessences:
2x Greater Quintessence of Might
1x Greater Quintessence of Alacrity

Glyphs:
9x Greater Glyph of Alacrity

Seals:
4x Greater Seal of Clarity
5x Greater Seal of Alacrity

Marks:
9x Greater Mark of Might

I chose this set of Runes for the high Damage, wich will make our attacks go to the sky with our Qs, we also benefit from high Attack Speed, what will help Vayne and Nassus, and reinforce Shaco, and a bit of Mana regen, since the three of them a early game mana eaters.

NASSUS
Obviously we focus on Q, but that's pretty much all, aside of that, you can balance between W and E, like I did, or focus on one of them, W as an Exhaust like spell, or E as a creep Mass murderer.
On combat against enemy champs, my normal sequence is R first, then W, then Q to go for the kill.
IMPORTANT:It is very important to farm Q a lot, remember that the most you farm it, the more you damage with it!

VAYNE
Again, we focus on Q, Tumble is not only an awesome spell for this build, but it also helps chasing, escaping, avoiding attacks, etc...
Secondly we level up E at level 2 to help running away, ganking and killing enemies, but without forgetting about W, since it help a lot too.
R is awesome to chase enemies, or een running away with Tumble, but, well, you already know Vayne ;)

SHACO
This one changes a bit, I'm not a big JitB fan, but it does help a lot.
The strategy behind Shaco Skill tree is leveling JiTB(W) at level 1 and then forgetting about it until level 14, why? Because when the match starts you gotta run to the golem, place all JitBs you can on the grass next to him, wait for him to spawn, attack him (not his sidekicks) and run to your JitBs, and pawn him, so you get a boost of XP, Gold, adn a nice buff, then go for the First Blood!
So, as said, on level 1 W, then forget about it until level 14, and focus on Q and E.
Also, you can notice I level E instead of Q in level 2, that's because it's great for harassing enemies, specially on the beginning, but remember, Q is our star damage maker!
Also, use your clone smartly!

This one gave me a hard time, so, well, if you don't like them, change them, but I think they are appropiate to this build since they help us on making massive damage, without forgetting who we are using.

FIRST ITEM
Choosing the first item is vital to help with our champion survivability so we stay in lane longer and die less.

FOr Nassus we'll get Doran's Ring or Shield, both are nice, though I prefer the Shield, since it gives us a little extra life and armor.

Vayne will be getting Doran's Blade, I really believe it's better for her early game than Vampiric Scepter, and with our runes It will help a lot on doing massive damage to get first blood.
Shaco will get the Vampiric Scepter, since he will easily get the First blood anyway and it will help a lot to staylonger in lane.
IMPORTANT: The first item is not a Core item, it is there to help in early game, but remeber to sell it whenever you need the coins to buy your next core item!


BOOTS
Choosing our boots is necessary, though you can prefer any other set of boots different than the one I chose.
Nassus should get the Ninja Tabi, other boots may also be nice, but this ones have a lot of Sinergy with the rest of the build, since they give extra armor and dodge which are two things Nassus really values here.
Vayne sand Shaco should get the Bersekers Greaves, since the extra attack speed really helps them.


CORE ITEMS
Everyone gets an early Sheen to augment the post-ability damage, that is the heart of this Build, this should be bought after the basic boot buying, but before the complete Boot.
Also, everyone gets a Trinity Force early-Mid game, which is vital to us for post-ability damage.

Nassus gets a Warmog as soon as possible, why is obvious, an Atma Impaler, then a Aegis of Legion/Force of Nature/Thornmail, and a Banshee Veil or Randuin Omen, . IF you prefer, you can go for a MAnamune instead.

Vayne will go for a Black Cleaver to sum evwen more damage and some attack speed, and her late game items are Guinso Fury Blade and Wits end, but these two can be changed for any other you prefer(consider a Thornmail if necessary), Last Item may be Stark's Fervor, a Frozen Mallet, or whatever you consider will benefit you the most depending on the kind of enemies you're facing.


[EDITED]
Shaco should go for a Malady, sonce it benefits him and his clone, and then the Trinity to get full hold of the build.
His last items may be Infinity Edge to augment even more the post-Q damage, and then Black Cleaver augment damage and speed and finally Madred's Bloodrazor, though if you need the power you can buy the Cleaver first of the three.

Nassus gets a mandatory Ghost, since it's his best way of escaping, in my experience, it exels Flash for Nassus. His Second spell may be Fortify, Rally or Teleport, any of these is fine.

Vayne gets the escape pack, Ghost and Flash, since she's really squishy. If you're confident and believe no escaping will be necessary, gat Ghost and Heal/teleport.

Shaco gets a mandatory Exhaust, which will help us get the First Blood as well as help eswcape and chase the rest of the game, and Flash for the Deceive-Flash escape combo.

Any ideas are welcome, constructive criticism only please!
And I hope I helped!
Also, I plan to make this visually better, but I still don't know how to make some things like inserting the item images, etc... :S