Have you ever tried making your own vocoder patch?
Do you understand how the vocoder itself functions- and how to take advantage of this?
Use pulse waves for clarity in words, saws to thicken the sound. Try to keep everything somewhat in phase, and feel free to dial in some noise for extra carrier functionality. Perhaps add some drive; run it through an open resonant lowpass and perhaps a high-pass right after to clear out some muddiness. Waveshape a little drive, compress the entire patch, use minimal modulation effects for clarity. Make sure your envelopes aren't doing anything funky unless you intend to hit a note with each syllable you speak/sing. It doesn't matter if the carrier timbre sounds like crap au naturale- it matters how it sounds when being used with the vocoder. Make sure to adjust the vocoder settings as well; shift your formant to taste, filter what you need to, and continually adjust for clarity.
Finally- of course a vocoder patch with the actual voice bled in or layered over will sound more clear!
Also, realize that not every robotic vocal effect is created via a vocoder- in the modern day, programs like Antares' Auto-tune and Celemony's Melodyne are frequently used to create perfect pitch, tweak formants and remove vocal vibrato. Linear-predictive coding, a precursor to some types of vocoding and a technology used in cellular phones, can also be interesting, with programs such as Sonic Charge's Bitspeek making for creative vocal timbres. Merely downsampling a voice, playing it back at a slower/faster speed or pitch shifting it while constraining rhythmic values can also result in a robotic sound.
http://soniccharge.com/bitspeek
http://www.celemony.com/cms/ (I heavily recommend acquiring a copy of this for easily manipulating rhythm and pitch of any audio)
http://www.antarestech.com/products/auto-tune-evo.shtml
Example Patch
http://dl.dropbox.com/u/1715217/shesellsseashells.m4a (the recording clips a little, my bad.)
Unfortunately I can't get the patch to you due to my current setup, but that was made by using Mono 2, three unison with 2 detune and a bit of spread (a bit too much, I'd use less spread). Oscillator one was a saw wave in unison mode with no detune or phase offset; oscillator two was a saw in sync mode 9 semitones up. There was a lot of noise added (almost full) and punch turned to around 70.
Filter 1 was a bandpass filter set to near-thru; with resonance turned way up, keytracking set to +1 and the cutoff tuned so that the resonance was the same pitch as the note. It was then run in serial to filter 2, a comb filter; Cutoff was at 60 with +1 keytracking, and resonance all the way up. Pre-filter drive was applied, about 1/3 of the way.
The volume envelope had the slightest bit of release, full sustain. The EQ was used to gain a lot around 800-1000 hertz and 1800-2400 hertz.
The vocoder was enabled; FcOffset was set to a fifth (+7), vocoder resonance was around 20-30, and E.F. Sensitivity was very little.[/b]