I've never done it, but if I were to, I'd obviously start out with good cider. I know some of the natural cider vinegars are unfiltered and thus have the bacteria, or mother still present to some degree. If you open it up to wild stuff, you will definitely end up with vinegar, but the quality might be suspect since some other things can get in and really funk things up. Really, you want as much cleanliness as in brewing (I would think) since you want the acetobacter to turn the alcohol into acetic acid. You don't want mold or other funky stuff growing in there while its going on.
I also know acetobacter is a big fan of oxygen. If you brew in a carboy, you could use the wooden dowel trick like a lot of people have done for making sour beers. Stick a long wooden dowel rod through a drilled stopper. Place the stopper into the carboy. In theory, this lets some oxygen in for the acetobacter to do its stuff.