Finally, Chirp seems to be working with the VX-6R. I've had no issues with the FT-817 or the ?Baofeng, but the VX-6R was just incommunicado as far as Chirp was concerned. I thought it might have been the cable, but a Chirp update seems to have fixed it. One quirk, though, is that when uploading a loadset to the radio, you have to hold the V/M key until after clicking OK. It should automatically switch from WAITING to RX once you click okay (and are still pressing V/M).

I've put the loadsets on Gitlab.