I still find it a difficult proposition. A microphone like a shure KSM 44 can pick up the sound of a pin hitting the floor. They are just too sensitive to try to use the monitors while singing. If you can hear the monitors the mic is going to pick it up too. The big problem with this is when you go to EQ the vocals, since the frequencies from the monitor will mix in with the recorded vocal. You'll find yourself trying to boost a frequency but your picking up a mix of vocal and monitor. I still like the headphone proposition.

I was thinking on this a little, you could try using a different mic for vocals. Like a SM57 works great, and won't pick up the monitors as much as a KSM44. Just a thought.