use a simple compressor. start with these ideas, hopefully they do the trick:
1. lower the "threshold" until you are reading at least 3dB of compression during the loudest parts. more is fine too, experiment. but remember, the compressor does nothing at all until you've exceeded the threshold. depending on yr compressor, you may need to switch yr meter to reading gain reduction instead of output.
2. start with a fairly fast attack. in other words, set it fully to the left, then up a notch or two. think 9 o'clock ish.
3. for release, it should be slower than attack. maybe start around 11 o'clock.
4. then set the output for proper level in the mix and give a listen. hopefully things even out a bit. but remember too that the above settings depend on the style of the vocal. ie, a crooner who holds a lot of notes vs a rapper who spits out words rapid fire. the best way to understand how a compressor works is to listen in real time while you move the attack and release around. you'll get it pretty quick as long as the threshold has been lowered enough to make the compressor start to work.
best of luck!
-s
