VWP has to be defined by whomever made up the term. PWM normally runs at a constant frequency and varies the duty cycle. Some systems have a constant pulse width and vary the frequency, but that would not be VWP. Maybe VWP is constant duty cycle and vary the frequency, not a very sensible thing to do.
The datasheet for the 555 oscillator/timer shows a Pulse Position Modulator that changes both the frequency and duty-cycle at the same time. Maybe that is also called Variable Width Pulse.
Are you possibly looking for info on the Ford / GM communications protocols used in their respective OBDII systems. If so, Google OBDII or OBD-2 and see what comes up.
Several sites discuss, some in great detail, the protocols and associated timing/signal types used.
Dialtone