Over the years we've found that for each 100 maintenance hours (tacho or airswitch) you get around 110 VDO hours. Costings done accordingly, so if you want to hire on airswitch then be prepared to pay about 10% more than VDO.
There are some VDOs that run on master switch, in which case the VDO time at start up is noted, sometimes the stude may have been sitting there listening to the ATIS for 10 minutes
while you do your secret instructors business prior to the flight...obviously they shouldn't pay for that.