I'm pretty sure most,if not all distortion pedals are engineerd for guitars.You may do better to for a vocal specific distortion contained on multi rack units like tc-helicons or even behringers if your down on cash.

The only distortion box I could point you towards would be an electro harmonix muff pi.Its used often as a bass overdrive as well so I wouldn't be surprised if it suited vocals as well.But you may really want to look into vocal specific units.

try out some stuff at a local music store.Who knows,the metal zone might work really well...at least you would have a great guitar distortion pedal even if it sucked for vocals(I'm assuming your looking for a 'demon of the fall' voice?).