Tacho time works of engine RPM. The tacho turns over 1 hr when the engine is running at cruise RPM for one hour. If the engine is idling or low power (such as on the ground) the TACHO time turns over slower.
VDO is typically based on clock time when the engine is running - run off oil pressure as the switching mechanism or something along those lines.
I am sure a LAME can give us a better idea of exactly how it works, but thats the basics of it as I understand it.