I checked the VIM2 with SPDIF from GPIO Pins 13(SPDIF Sig) and 14(SPDIF GND), to my Onkyo HT-R391. This works, but was able to get only two channel audio. Firmware is VIM2_Nougat_V190518. Oddly, with the Tone Board mounted to the VIM2, SPDIF output did not work. I have no idea why.
User and ROM developer @superceleron has made some Custom firmwares for the VIM2. This ATV custom ROM, it is said passthrough(DTS, DTS-HD etc) works. He also offers some custom regular Android ROMS, see here.