Just a quick note on those making vinegar from commercial beer. Of course if you want to make a large quantity, kegs are the economical and efficient way to go. I have experience (as have some customers) with kegs though and the lack of cleanliness in many kegs, especially those on loan from beer stores, can often ruin your fermentation with mold.
In short, if you own a keg, make sure you fully clean and sanitize (i.e. with dilute bleach) in order to flush out and kill all mold spores. If you can’t guarantee a store borrowed keg is clean, you may be better off with cans or bottles.
Not taking this care will find your vinegar fermentation ruined by mothers that become moldy before the vinegar is complete.