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.
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