Figure-8 mics are your friend here.
A figure-8 on vocals with the dead plane of the mic pointed at the sound hole on the guitar will give you great isolation on the vocal. If you're getting too much room echo on the back side of the mic, a gobo or small acoustic baffle stuffed up to the backside of the mic can ease that.
If you want to mono-mic the guitar, another figure 8 down there with the dead plane oriented to the sound hole of the singer works fine. Stereo micing can be a little trickier, but I've done find with XY, and MS.
The main thing you want to avoid is significant coloration of the other source in any one mic. I tried the baffle thing between mics once and ended up with colored vox bleed in the guitar. It was light, but the off-color sound was hard to mix into the vox mic.
You're much better off with good sounding bleed, so when placing your mics focus on only on reducing the bleed, but making sure the bleed that's there sounds OK.
a cut and paste from another forum long ago
In later sessions I learned that the round baffle is much more effective just a few inches from the back side of the instrument mic. Here is a track from the completed CD: http://www.cheap-tracks.com/mp3/chea...d_o_sample.mp3
Here's another clip http://www.cheap-tracks.com/mp3/kiser_guitar_sample.mp3
This one done with XY AKG Blue Lines on the guitar. A little color in the VOX from bleed in the instrument mics, but overall a pretty good result. The guitar mics were pointed downward a fair bit to minimize vox bleed.
The LD mics in all of these clips are Studio Projects C3s.