Tacho time isn't the same as VDO. Depending on what you're trying to achieve tacho time can be advantageous compared to airswitch.
There's a range of ways an operator &/or pilot can determine times for charging, maintenance & (pilot) log book. Maintenance typically must log
at least the time in the air. Pilot logging is from when the aircraft 1st moves under power to when it parks after the flight. Charging can be based on anything the operator wants eg time in the air, time the engine is running, number of miles flown, mate's rates or whatever.
VDO is a brand of hour/decimals of an hour recording meter. Hobbs is another brand. The ubiquity of these brands means the brand names are pretty much interchangable with 'hour meter'. The hour meter can be wired to start recording in a number of ways. Common one's include:
* master switch: the meter starts as soon as the battery is switched on.
* oil pressure switch: when the engine starts the rise in oil pressure trips a pressure switch that activates the meter
* air switch: a vane or pressure sensor trips once a predetermined speed is reached during the take-off roll eg 30kts
* gear switch: the meter starts when the gear is retracted & stops when the gear is extended again
* squat switch: the meter starts when the weight comes off the wheels during take-off & stops once the wheels are supporting the aircraft again.
Air switch, squat switch & gear switch are pretty much synonymous with 'flight time'. Gear switch is slightly advantageous but not much (unless you fly the entire trip without retracting the wheels....
). You don't pay for taxy time, time to do pre take-off checks etc
Oil pressure & master swtich wiring means you start paying whenever the relevent switch is activated.
Alternatives include clock time where you (or someone) writes down time(s) from your watch when you start & stop the engine or move after start to parking after flight or take-off to landing.
Another alternative is to use the hour meter built in to some tachometers. This will be usually be calibrated to clock over one hour at some typical cruise rev. If the engine is at less than this power setting then the meter runs slow ie you get more time for every hour it records. If the engine is run at high RPM then you will get less time for every hour it records. For typical 1 hour training flights the tacho will records about .7 to .9 hours per clock hour of engine operation. You can save quite a bit this way. The difference will reduce as the proportion of time spent at cruise RPM increases. If you're only trying to build time then tootling around at a low power setting will save you quite a lot of money.