Pretty nice guide, but you should make it more attractive (put some pics, maybe even videos, you know, stuff like that).
The only thing I disagree is that Vayne counters Sivir (i'm sayin' this from my own experience). I'd say that they have equal chances. Although Vayne's late game is much stronger, in early game Sivir will have higher damage output with her AA+W+Q combo (if she manages to land it), and if someone is good with her, she can easily beat Vayne IMO.
The rest is completely fine. +1

The guide is good, though the item section could be improved.

Also, the title states it is an ADC guide Vayne.
Make it more interesting :P, to attract readers.
It is kind of obvious that someone plays Vayne as an ADC :P

The guide in itself is not bad or anything, but you have relatively poor formatting (a few walls of text, grammar). That isn't really grounds for a -1 though, so keep it up.

One thing that seems strange to me is the late dorans blade. The extra bit of lifesteals nice, but I think itd be better to buy a longsword to build into last whisper.
Still, a good guide :)

good guide +1