Hello. I'm a Silver player who loves playing Vayne and has a lot of games on her. I thought I would share some opinions on how I think Vayne should be played.

What's so good about Vayne?

Insane Potential Burst with crits and her W.
Incredible duelist late game against most champions.

How can Vayne really suck?

Range is pretty short compared to Caitlyn and Tristana, who can usually demolish her.
Extremely low base HP early and late.
Horrible if you don't have any idea how to play her.

So About Them Items........

Why don't you run Ghostblade?

Well...
I've found that trading Ghostblade for Phantom Dancer really cost you a lot. You lose lots of attack speed and some crit. Also some passive movement speed it found on few other items. Youmou's temporary speed and attack speed buff and dmg/armor pen isn't worth all the other stats you lose for more time.

When Do I sell boots?

Boot selling is only when you really need the extra stats your last item plans to provide. Even grabbing zephyr in place of your boots will cost you good movement speed as well as any boot enchantment (base defense, homeguard). However you must decide when it is needed.

Flash and Heal are basically always run. Against high CC teams Cleanse can also be effecttive. Barrier has nothing on heal as its only for you and is now less strong compared to Heal.

Gameplay is based on a lot of experience and learning. Simply playing her will give you the best idea of how YOU play Vayne best.
Reall the only way to break it down is:

Farm if you have a passive support.
Follow and fight with a more aggressive support while also being sure not to go too deep.
Mid to Late game focus on following team during teamfights and looking for Condemn picks in the jungle.