VDO or Hobbs is a clock that runs on a meter usually activated by oil pressure but sometimes by master switch. This is the time that you log and that the instructor gets paid for if casual.
Normally you get around 110 VDO or "revenue" hours to 100 tacho hours and costings will be done on this basis. So, if you get it on tacho not only will you log less but you will pay 10% more.