Terence,

I wasn't aware that the unit
automatically switches from VLOC to GPS or vice versa.


Me neither ;-)

It switches from GPS to VLOC only, on the condition that the VLOC
frequency is active, once you are kind of established on the final
approach course. Also, it switches "gradually", so as to not make the
autopilot go berserk.

--
Thomas Borchert (EDDH)