What's the difference between PWM and VPW?
The word 'width' in Pulse Width Modulation is a bit misleading. PWM actually encodes an analog value as the ratio of High or On time ('pulse width') to the total period of a rectangular waveform. 

VPW is another name for Pulse Length or Pulse Duration Modulation (PDM). It encodes an analog value in the absolute width of the pulse. The period between pulses is unimportant. 

PDM is used in RC servos, which typically respond to a pulse width varying from 1 to 2ms. The nominal repetition frequency is 50Hz (20ms between pulses) but most servos will work between 40~70Hz - and some go up to almost 500Hz (the theoretical maximum repetition frequency of a 2ms pulse).