What am I doing wrong with my Hard Cider

ALL IS NOT LOST!!!!

I had a similar problem with a distinctly vinegar flavour to my apple cider having left it maybe two days too long in the fermenter.

With Scottish blood flowing through my Kiwi veins, I couldn't stand the thought of it going to waste, so I boiled 2/3rds of the cider-vinegar to 70 degrees C (killing just enough yeast to sufficiently ferment the added sugar from the apple juice to the point of carbonation and no further), then added 9 litres of preservative-free apple juice.

I prayed and I trusted, and even only two weeks later have enjoyed the finest cider I've made to date (further enhanced by watching the All Blacks beat our arch-rivals, the Wallabies, in Yokohama as I did it)!

If the cider is really turning into vinegar, than you've got a bacterial infection, probably acetobacter. This bacteria will metabolize alcohol into acetic acid. Acetobacter is present in small quantities in apple juice. It's also carried by fruit flies.

There are three things you can do to fix this.

1. Observe proper sanitation technique. Anything the contacts the juice or the finished cider should be free of contaminants like bacteria, mould, or wild yeast. Palmer's How to Brew has a good section on sanitation.

2. Dose the juice with potassium metabisulfite 24 hours before pitching the yeast. This shouldn't be necessary with pasteurized juice, and a lot of cider makers (myself included) skip this step even with raw juice. The k-meta will reduce activity from bacteria and wild yeasts, given the brewer's yeast a head-start at becoming the dominant micro-organism.

3. And it goes without saying (but I'll say it anyway) -- use an airlock during fermentation to prevent any airborne contagions (including flies) from getting at the juice.