Hello John,
You're correct in that the DVSI AMBE chip is only needed for the Vocoder section but it wouldn't surprise me if many implementations EXPECT to see the AMBE chip there even if it's not being used. The DIY market for Dstar is very robust and with various GMSK modem boards out there now (even all SW versions too). I don't see why this wouldn't work but you might need to work with the GMSK modem vendor to get the SW to work properly.
--David
KI6ZHD
