Just to stir muddy water. VDO is a brand of
instrument. Hobbs is also a brand of
instrument.
Either brand of gauge can be wired to run when:
1. The battery master is turned on.
2. The oil pressure rises.
3. The gear is retracted.
4. Weight on wheels (squat) switch.
5. Airswitch.
The point is made above that the rate you pay will reflect the method used.
Saying something is charged as "VDO" does not define the method used to switch the meter on.