Twitch player:
I get bonus MS during my minor invis, and drastically more if im approaching an enemy champion within a narrow band.
Vayne gets about the same MS, and drastically more for the duration of her R, at all points in the game and Sivir gets it for landing any skillshots or autos. I've had a Vayne chase me from mid inner turret on purple side to south side red buff and she was outrunning me the entire time.
The only way Vayne is going to be outran by other ADCs is Miss Fortune's Strut being up and maximized, or someting like Ezreal constantly blinking away or Cait/Ashe impeding her movement (even just placing a trap can sometimes force Vayne to walk around it, letting Caitlyn escape to safety.) and even then, I have both as and against vayne, numerous times, chased them down anyway. Vayne is very very tough to walk away from and very very easy to chase down.