If you've recorded your vocals the way most people do, then they will be mono anyway! If you want to hear a single mono track in both channels (assuming that your headphones are working normally in the first place), then you either have to pan the mono track so that it feeds into both sides of the master channel, or you have to duplicate the vocal so that it occupies both sides of a stereo track. The former is the preferred way of doing it - have a mono vocal track in your stereo mix and pan it to where you want it in the stereo field.
As far as monitoring during recording is concerned, then there could potentially be a few other issues - it rather depends how you are feeding the channel out of your sound device. It's not quite so easy if it's a mono track to feed both sides of a pair of headphones when you haven't gone via the master mixer - IOW you are using a direct output from the track. That would only feed one channel by default. But if you are going via the mixer (which you normally would whilst laying tracks) then it should just be a matter of panning the vocal channel into the centre.