I don't have a Kinect, but I would guess that it would be whatever word it is in the "Power" section of the menu. Have you tried saying that word or phrase?
from what i have tried there isnt a command for the kinect that allows you to turn into the vampire lord form it didnt work with beast form (werewolf) either seems the kinect commands only recognize dragon shouts when it comes to abilities i may be wrong but i have never got anything but dragon shouts to work
The time has come for you to decide, if you want to leave it all behind,
to open the gate into my world, will you follow me?