The compressor has a very short attack time, very useful for lead guitar- not so much for vocals. It isn't a matter of quality, the thing is great for what it was designed for, but it won't do what most people want on their vocals. Anything is fair game if you are looking for an effect.
I bought the MXR Dynacomp pedal new about 1999 or so. My pedal had a long attack time (maybe a 1/4 to 1/2 sec). It was long enough to have a noticable drop in volume at the beginning of a guitar phrase. I could only use it if:
1) I kept a significant guitar hum going to keep the release from kicking in between pauses.
2) The compressor knob was at 9:00 or less.
There was a lot of talk about the long attack on this pedal at firebottle.com and some mods to lower the attack time for lead guitar. The mod was involved and not really worth the trouble.
The old ones were different. The bottom line is that there is no control over anything you need to tweak to compress your vocals well. If it worked, it would be a fluke. The other thing is that you have a possible impedance and level mismatch. It was designed to accommodate a guitar pickup, not a line level signal.