Vayne Building Youmuu's Ghostblade

Can I ask where this came from? Last time I saw a lot of Vayne play she built BoTRK and then went around destroying people with the scaling health damage but lately all I am seeing Vayne's do is build Youmuu's Ghostblade first and then have difficulty killing a Nami while I tank 3 enemies for 10 minutes. End of game damage seems to confirm it as bad when Vayne is doing less damage to champs than everyone but the supports inspite of having a frontline that let's her do all the damage she wants.

So what gives where did this Youmuu's rush come from and why do I see so many people doing it?