The whole The Acoustic Verses album by Green Carnation is amazing... Though if you need simply a guitar and vocals, I think the best choice from the album is The Burden is Mine... Alone. Though it's a beautiful but very sad song, and I don't know if that's what you're looking for.
Another option is Anathema - Temporary Peace. They once had a video on their site of Danny playing it just on an acoustic guitar and singing so it definitely works that way.
And it's an absolutely amazing song.