.... (we'll use an acronym from now on) has four inputs.
VPM can either modulate channel 1 with channel 2 "mono mode", or in stereo mode, channels 3 and 4 modulate channels 1 and 2 respectively, and 4 channels of incoming audio are processed.
There is an additional switch that Inverts (flips) the inputs.
Some DAW hosts allow multichannel routing, so to get full use of the plugin, it's good to know your way around your DAW's internal routing.