LVDS difference between Vpp and Vdiff
The differential signal voltage \$v_{diff}\$ is the difference between the positive signal and negative signal. It is a time-varying waveform.
The peak-to-peak voltage \$v_{pp}\$ is the difference between the maximum value of a signal (in this case \$v_{diff}\$) and its minimum value as the signal varies in time. It isn't a waveform. It's a parameter describing a waveform.