I haven't found one that sounds as good as SonicCharge's Bitspeek, but unfortunately that won't use an external carrier--it's just always a saw--but it tracks and replicates formants very well. It uses tech called Linear Predictive Coding.
"LPC Vocoder" from BrainDoc.de (defunct) uses this but in FL Studio it makes this awful buzzing noise and it's either because of buffer problems or routing issues because if I change the buffer size from Fruity Wrapper's (vst loader in FL) settings the buzz will change, but also if I adjust the routing options in FL and send everything but "Mix/Voc" to output 2 (which sends it to Send channel 1 in the mixer for ??? reasons) and mute that it'll sound good.
very nice torrid gristle! oooh you sound so AI when talking that talk. i heard a real talkbox can bust your teeth out if you crank it too loud. and you can make one yourself out of common household items. if one were so inclined.