This is a Vayne build like all the others, just with small changes, i have been playing this build a few times now, and in all the games i had a good score and could easily ambush one from their team and kill him. another thing i like about this build is that you can survive a 1v1 frontal fight with another dps. The extra life from the mallet and the lifesteal makes you survive a bit longer, and the E(crossbow) and Q(tumble) will do that they almost wont touch you before they are dead. You have to be careful tho, vayne is an assasin, not a hero who can survive a 2vs vayne gank, stuns and snares can be very painful because you depend on the lifesteal.
First i start with boots and hp pot, i like the runspeed in the start, espcially because vayne is squishy, so it is easyer to run away(and hunt) in early game. afterwards you build zeal, gives you extra runspeed and gives you atk speed, atk speed and silverbolts goes hand in hand, so you should get a high atk speed to use silverbolt the most. after this you need lifesteal, if you dont have the money for full bloodthirster you should buy lifesteal first. after bloodthirster you go for phantom, then madreds(wich is build like this: bloodrazor - recurve bow - pickaxe) the rest of the items you can chose for yourself, i like frozen mallet because of the slow and the health boost wich is nice, because you won't get killed by 2 hits now. the last item i like to buy is last wisper because of armor pen and a nice atk speed boost. but the last 2 items are dependant on the game.
This skill build is very nice i think. first you get tumble - a very nice harassment skill, and can be usefull for firstblood because of the extra damage. then you pick the crossbow, if you play good you can tumble, then stun them against walls, and bam! they are dead. this is a good combo, and if you wont kill them on 1,5 sec, you can tumble after them and kill. silverbolts will first be very useful when enemies start to get life and you get atk speed, so from early mid game(10 min) you can start to fell it's use!
in team fights you stand back, wait for your tank to engage, then activate R, shoot them tumble close, but not into the fight, maybe tumble to the side of the fight so you can hunt low lifes fleeing. the most important is that you stay out of close combat, and it is easy - vayne is ranged.