Also, to add: there should be no difference with the VST3 version, in practice. If there is, the Host-XL module of VCV isn’t currently mapping the input channels correctly for VST3 plugins. Just to make sure, I now installed the VST3 version of AudioThing Arguments, and hosting it in Reaper, routed a signal into channels 1/2, and another signal into channels 3/4. Nothing more needed, the signal routed to channels 3/4 appears as the sidechain signal within the plugin - just like in the VST2 version. Can confirm this doesn’t work in VCV, and the VST2 version of the plugin is needed.