ENR 1.1 - 37
19.4.3 states When using radio navigation aids as the primary means of navigation the aircraft must be navigated by reference to the aid which provides the most precise track guidance with wich the aircraft is equipped and the pilot is qualified to use.
The oder of precision is Localiser, GNSS, VOR, then NDB.
my interpretation is that, if you have VOR and ADF onboard, qualified on both, then in the above example, you'd have to use the VOR not the NDB, correct?
That doesn't pass the common sense check. If that applied to an instrument approach then you wouldn't be able to do an NDB for currency if the ILS was available.