EQ on vocals can be a tough thing sometimes. Vocals are in the same league as acoustic piano,
acoustic guitar and violin, they don't take a whole lot of EQ very well. There is no good recipe or guideline here other than use your ears and find what's needed. A dozen small bumps and cuts may suit the vocal better than a couple of huge peaks and troughs do.
Instead of trying to force-fit the vocals via the wedge of EQ, the right mic/preamp selection can be all-important in getting vocals "right". I'm not saying that you have to have ten grand worth of vocal mics and another ten grand of preamps (though wouldn't that be kinda neat?

). But if you have more than one mic that can be used as a servicable vocal mic, try the vocal through two or three mics matched with the best pre (if you have a selection; if not, that make your choices a lot easier).
What is a great vocal mic for one person may sound veiled or colored on another person. And no EQ is going to fix that quite right, it just does not have the resolution.
Perhaps it's too late for your current situation, if you can't go back and re-track the vocals. But it's something to definitely keep in mind for the next time, or if you do have the chance to re-track the vocal. You might find real quick that there's one mic that works for singer A and one for singer B and getthat issue fairly settled pretty quick.
G.