It’s not clear to me what a VST Instrument Sidechain would do/ be.
Sidechaining is used primarily for compression and gating … although someone could creatively probably find other uses, such as delay, for an example.
If you want to use the VSTi output to trigger … then just select its output in the mixer and use a send to the track where your sidechainable plugin is placed.
If you want to use another sound source to sidechain trigger to the output of the VSTi, then place the plugin in the effects chain on the VSTi output channel in the mixer and now go to the other audio source and send back to that plugin from the other source’s sends.
Probably I’m missing something … but it’s not clear to me what having a sidechain option in the VSTi could do beyond this anyway.