If you go to the award availability search page on EF - https://www.expertflyer.com/air/awardUpgrade.do - and you select VS from the drop-down menu, there is a listing of G for Upper Class, P for Premium Economy and A for Economy. I would normally regard that as a reliable indicator of the booking classes used by the selected airline, at least for full redemptions (I know that different airlines do different things with upgrades, but it sounds like that's not what you're interested in here).