Phage gives lane dominance. It gives you health, ad and a punishing slow.
It's not really an overkill since early levels Vayne isn't that good of a chaser.
It also builds into nothing that Vayne needs
Why not take a HoG and farm (If you really heed the tankiness)? You're going to be passive anyways, and not engage if possible. HoG helps you get to your full late-game build faster and not die.
© 2014 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.