Acceptable range is 0-3: 0 is the default sine, 1 the square, and 2 the rampdown. Protracker docs speak of 3 being a random waveform, but I've yet to see it implemented ! Neither pt1-1, pt1-2 nor pt2-0 implement it. You're supposed to add 4 to control whether the viboffset is reset at each note (if 4, not reset, but this looks buggy as hell to me...) See Vibrato, for the waveforms used.