phi is the speed of the vibrato and plow the depth of the vibrato.
Null values for phi or plow just mean `keep the previous value.' 
The basic vibrato waveform is a sine waveform.
It can be changed to a square waveform, or a rampdown waveform.
Here are the actual shapes used, with origin et all.
    ****                     ********                     **
  **    **                                              **
 *        *                                           **
*          *                                        **       
            *        *                                          **
             **    **                                         **
               ****                  *******                **
   Sine 0                     Square 1                Rampdown 2
(see Set vibrato waveform, for a detailled discussion of some aspects of that effect).
The resulting waveform has an amplitude of 2*depth, e.g., its values are in the range [-2*depth, 2*depth], and its full period is 64/speed ticks.
The position in the waveform is reset at each and every new note on the channel.
Scrolling mode:
1 A-4 vb 5/3
sample number / note / depth/speed.