I'm not sure what it is on the xbox, but on the ps3 it was originally the select button the controller so I would guess it would be the back button the xbox one.
the only reason I know of that it was an issue on the ps4 is because they made the select button slot share, and added the touch pad button and harmonix didn't realize they could map to that instead.
I am sure someone else might know. but I know I am in the minority of people who play vocals.
Can confirm from previous attemps that view button does function for overdrive on XOne.
Also A button for tambourine/cowbell hits (should work as X on PS controller)