Exactly.
This problem is so many-fold, that it's almost impossible to answer in a thread.
First off, no spesific EQ-advice can be given based on a source, no matter how defined it is. It all depends on, well, everything.
Figure the source, the mic, applicable processing, the PA, the acoustics, the context and desired result into the equation, and do the math. There is no one answer, as a change in any of the above could throw the whole thing totally off.
So, what to do, and how?
-Start with micing close enough to the source to eliminate unneccesary bleed from the rest of the band. Lips touching the mic, that is.
-Find the best mic for the vocal sound (no EQ). Expect to try many. Pick the one that has the best compromise of nice sound, least bleed and least feedback. Maximum volume w/o feedback is your goal.
-Mic-to-speaker -orientation! Any speakers re-inforcing the vocal should be facing the dead-end (the null) of the mic.
-Apply as little dynamic processing as possible. Lay off the compressor. Remember, we want a better gain-to-feedback ratio.
-Make sure you have a good EQ unit available. Preferably parametric, as many bands as possible (four should do nice). Find the ugly spots (the ones that sound bad, or feed back) by sweeping (on a PEQ, with a narrow Q, do a SLIGHT boost, and sweep the freq knob to find the ugly, and then cut on that freq). Use only cutting EQ, as that will sound better on any sources whose sound you want to keep natural. Compensate later for unity gain.
-Expect to "make room" for the vocal with the rest of the instruments. Cut down on volume.
This is the very brief answer. Experiment, have fun. You'll learn in the process.