Forgive me if this is simplistic, but my first thought is to do basic strums while words are being sung, and throw in some embellishment (a la lead riff) in the vocal pauses. It may be a few notes or a few measures or even for a number of lines of the song (taken as a break, for example). The idea would be to not have the voice and the guitar competing with each other. From that point, one might choose to do fancy picking simultaneously with the singing for certain (and few) selected passages, to give them different emphasis.