Always a SLOW attack and a QUICK release.
I adjust the release to the tempo of the song, but the slow attack is critical for a pristine vocal. It depends on the type of compressor your using, but for years I've LOved 10:1, but other settings are cool too, it just depends on the musical content and what your going for.
An LA-2A is considered one of the BEST compressors for a vocal, and they do not even have a fast attack adjustment, they're real slow.
I use my attack knob more as an EQ, if it's set at a slow attack 50ms+, start turning it slowly becoming shorter and shorter and start listening to it as an EQ. If a vocal has a lot of sibilance, I will start shortening the attack and it starts shaving off the sibilance.
It always amazes me, that 25 years after I first started using them, I'm still learning new things about them all the time. They only have about 3 knobs, but I think I will always be learning new ways to use them ... or not use them.
Russ
Nashville