I'm guessing that you want the vocals to TRIGGER the sidechain of a compressor. I've never used this method (lack of correct gear), but I can see where it might be useful.

There was another thread that discussed the concept behind sidechaining - there's good info there.

Here's an example of a possible application for vocals:

On a loud rock guitar mix, you could subgroup the guitars, run them through a stereo compressor, and then send the vocal (via an aux) to the sidechain input of the compressor. Then mess with the parameters so that when there are vocals in the mix it pushes down the guitars a few dB to make room for the vocal to come through.

I will often try to achieve a similar effect by automating the guitars and vocals, but using a compressor could probably wrap the guitars around every vocal phrase a bit tighter and it won't be such a pain in the ass. Actually, I would guess that the best technique would be to combine the two methods - compress the guitars a bit (not too much), and ride them when necessary.

Also a way to do it is as Fletcher described a similar one in his website - you take all your guitar tracks, sum them to mono, compress the shit out of them and bring them up on another fader, adding just a little bit of body to your guitars. Then use another compressor with a sidechain input to duck out that guitar witha a vocal feeding the sidechain.

I am pretty sure that this is exactly what has been done with Tool's latest album Lateralus. It has one of the heaviest and most powerful guitar sounds ever and most of it is a single guitar playing only once and panned center, yes, center on top of rather quiet, narrow, thin and buried vocals; bass; snare and kick. You still can hear everything really well. Even vocals. I wondered a while - how did they do it?

Well, they "cheated" a bit imo... The guitar definitely is played only once but tracked with multiple amps and multiple mics and they are panned so that they appear as a rather wide cloud between the speakers. The vocal is dead center, if there are any stereo effects on it, the mix effectively buries them. The guitar is as center, but appears just bigger and wider. And every time the singer does anything, the guitar seems to lose a bit of it's definition in center - you actually don't hear it casually listening, but if you listen real closely, every time a vocal phrase comes up, something disappears in the center...

My bet is they did something like that over there...