There is 3 EVOC in Logic : The EVOC 20 PolySynth virtual synth, the EVOC 20 FilterBank and the EVOC20TrackOscillator. For the purpose of my answer, we will be using the Software Instrument version of the EVOC20. Steve H. covers the EVOC TrackOscillator and the EVOC FilterBank in great details in chapter 2 of the 202 Logic's Plug-Ins Unplugged tutorials.
Basically the EVOC, like any vocoder, analyses the voice (or any other audio material) and splits it up in frequency bands (from 5 to 20 in the case of the EVOC 20 PS). This is the "modulator" and it modulate the carrier, usually a synth pad sound.
Here's a simple way to begin using the EVOC 20 PS :
1-Drag an appleloop on audio track 1. You can use the " House Music Female Vox 02 " from the Jam Pack Remix tools that is installed with Logic Studio. Turn on Cycle Mode or make it loop.
2-In the Mixer, mute audio track 1. Don't do it in the arrange Track Header or it won't work.
3-Click and hold the I/O slot of software instrument 1. Select EVOC 20 PS / Stereo.
4-Select "Audio Track 1" in Side Chain of the EVOC 20 PS window. Audio track 1 is now the modulator of the EVOC.
5-Select the Vintage Vocoder / Atmospheric Pad preset.
7-Make sure the EVOC track is selected in the arrange and play some chords on your keyboard while the song is playing. The notes you play will have the "House Music Female VOC 02 " sound characteristics.
Don't forget you can use different audio material as a modulator, not just vocals. Drum loops makes really cool rhythmic pads!!!