Vboxmanage is in the folder that was made when you installed Virtualbox - the program itself, not the guest that you made afterwards. The path you added to your system path is to your guest, not the Virtualbox install folder, likely "C:\Program Files\Oracle\Virtualbox" (you'll need to hunt around for it). The Virtualbox GUI window that shows your guests and their settings runs from the same folder as Vboxmanage, so you can get the correct path from a shortcut to the Virtualbox GUI.
running version 3.1.2, 4.2.16, 5.0.14
Human government is like that crazy uncle who hides a quarter in his fist behind his back, then asks you to guess which fist the quarter is in...
No matter which side you choose, Left or Right, both Sides are empty.