Two Mics (or three if you you want to mic the guitar from two separate vantage points like the lower bout and at the end of the fingerboard [12th fret]) Give yourself some distance from each other to help avoid sound bleeding onto the other track although in this situation some is inevitable. One mic to each track input and away you go! This allows you to change levels and add effects to either vocals or the guitar without to much influence on the other. For better separation, hang a heavy towel or blanket between you and your vocalist with only eye contact available. You get that kind of connection which I think helps in live settings. My 2 cents worth.

Kudzuman