I'd use at least two mics... one on vocals and one pointed at the 12th fret of the acoustic if you want to keep it simple. Beyond that it will be difficult to get anything clear and lifelike. I suppose if you have a good condenser, you can put it at mouth level and angle down towards the guitar at about a 45 degree angle... so you still sing directly into it, but it's pointed at roughly the 12th fret.
Or if you have a sweet sounding room... you can just set up a mic somewhere in front of you, but I doubt you have a sweet sounding room.