Many people think you should get madreds early to play off of vayne's W spell but the truth of the matter is that Vayne is too squishy to get off that many hits. She needs crits... The way that I came up on this build was that I forgot to change my runes which I had set for Tryn. So I figured I would go critical strike and it worked very well. I had played Vayne too many times to count the old way and have never gone back. At the builds completion a Q crit does well over 1k damage making her very tough to deal with if played correctly.

The reason I run Ghost Flash is because doing this kind of damage and being so squishy you should be focused over almost any champion in the game. It is vital to have one of these off of CD to escape with any frequency. Obviously stay back from team fights and let a tank get focused before you roll in, and when it even looks like someone is going to come at you flash back and continue mashing your Q spell (don't wait for them to hit you because with Vayne it is usually time to run after you get hit and you want to be raining in crits for as long as possible). For a gank or 1v1 you should be hidden in a bush then ult Q out to position your E stun. When the enemy's stun wears off a good portion of the time they will be heading for the hills. They are right where you want them.

Variables are probably the most overlooked thing in builds. Change any build to counter what your opponents are doing because its impossible to make the end all say all build.

First if there is a single stun in the game I go Merc Treads, but even with them if you get stunned you can pretty much kiss Vayne goodbye.

Second if the other team is really tanky you have to go Madreds much sooner

Another thing to do if your opponents are going armor is to sub in a Black Cleaver for the Thirster and take it right around mid build