I am looking for a good, medium price, acoustic amp. I need to feed my 12-string (by mic) and my vocal into it, with an audience of 200 (on a good day). I would like a USB out, but ti's not a deal killer, quality is!
I've been a huge fan of the Fishman Loudbox series for this kind of thing, although my experience in working with guitars with them has been with pickups. They really make a pickup acoustic guitar sound like a guitar again, I'm not sure how.
I can pretty much predict that you may have some feedback challenges with a mic on the guitar trying to cover an audience that size unless you are able to get the amp out in front of you.
If you're doing the miked guitar thing, along with your vocal, you may want to consider a straight powered PA speaker on a speaker stand, like the QSC K10. It has two inputs, so you can run your guitar in one, and your vocal in the other. This will definitely get you more projection and flexibility than a straight acoustic amp, although it lacks some of the nice guitar-oriented features of the Fishman, and will cost more. Once again, if you're going to stick with the mic on the guitar, positioning of the speaker is everything if you are going for lots of gain to cover a sizable crowd.