The VCA/Mixer module is now finished!
Aside from the pull-down resistors previously mentioned, it was pretty easy to assemble to the panel. There's only one trimmer on this module, which is the CV Reject for cutting down on CV bleeding to the output. I sent the VC inputs an audio signal from a VCO, and could eliminate most, but not all, bleed, by turning it fully CW. Everything seemed to work fine in testing. Note that this uses linear pots, so you may not get the typical response you may expect from VCAs that use log pots.