are you setting it up the right way around? you obviosuly need to sidechain to an audio channel to have the vocals, and then you set up the vocoder on the midi channel and use the midi data to create the keys you want the vocoder to speak in. as far as getting a good sound, i've only ever really used the pre-sets (i use logic 6 pro, not sure if 5.5 has pre-sets) so i couldn't really help on that point