Recording a live acoustic guitar and vocal performance can be a bit of a crap shoot. Depending on the guitar and how the singer projects, you can be looking at both having too much guitar in your vocal mic or too much vocal in your guitar mics. Mic placement might improve things but you could still end up with a really cut down set of options when it comes to mix time. The EQ you need to make the guitar sound great might also emphasise a part of the vocal that doesn't sound pleasant and so on... It can be a tough balancing act of compromise after compromise. Here's a mic technique I learned from Steve Albini at his Mix With The Masters seminar, which gives fantastic isolation.
You'll need two flat bodied omni-directional lavalier mics - Steve uses Crown GLM 100's, but they're discontinued.
I'm using a pair of Countryman's omni-directional Isomax 2 mics, and they seem to do the job as well as Steve's Crowns.
Using thin double-sided sticky tape, secure the first mic to the soundboard of the guitar against the soundhole side of the bridge, with it's capsule window below the mid-point between the low E string and the A string. Stick the second mic directly below, with the capsule window located between the B and high E strings. For a steel stringed guitar, you should have the capsule window facing down onto the soundboard, for nylon strings, have it facing out at the strings, as pictured above. Secure the mic cables with some more tape so it won't be in the way of strumming - use good low tack stuff like Frog Tape that won't leave sticky residue on the guitar.
Using the omni-directional mics against a boundary in this way effectively turns them into Pressure Zone Microphones. You get a really flat phase response up to really high frequencies, and because the mic is so close to the source, the amount of ambient sound that ends up in your signal is incredibly low. Result? You get a guitar recording that has so little vocal spill in it that you shouldn't have any real issues mixing it.
Now, because we're using very small diaphgram condensor mics, you're not going to get a huge bottom end. With that in mind, place a Figure 8 ribbon mic about 5 or 6 inches from the soundboard, pointing at the centre of the soundhole. Steve used a Coles 4038, but I've had good results with the far cheaper X-Audia Beeb Reslo. The Figure 8 pattern of the ribbon mic means that the vocal is mostly in the mic's null - you won't have a lot of unmanageable vocal spill in this mic either, and it will fill out the low end body of the guitar sound.
Steve used a Microtech Gefell m930 for the vocal, slightly below the singers mouth, angled up at it, and the fixed cardioid pattern of the mic gave good rejection of the guitar. I've used my Neumann KMS105 for this purpose, with it's tighter hypercardioid pattern giving really good rejection while still capturing a great vocal sound, but careful placement of your vocal mic of choice should get you a vocal that doesn't have a bucket load of guitar in it.
This is a really great technique if you're doing a fully live recording, and while not everyone will have a pair of suitable lavalier mics just lying around, if you record acoustic artists on a regular basis, they're worth the investment for the results they'll give you. One last practical comment: In the picture above, I didn't have any double sided tape, so muddled through using just the single sided Frog Tape...this presented some problems with the mic lifting, hence mess of tape you see in the picture. I'd recommend putting a strip of the low tack tape flush against the full length of the bridge, then using double sided tape with a higher stickiness to secure the mics in place on that strip of tape...it'll be cleaner, more secure, but you still won't have to worry about residue on the guitar top or a chemical reaction between the adhesive of the tape and the guitar's finish.