Edited by Callimom - 2/11/12 at 8:31pm
I've had the same thing happen to me - buy an extra jug, either for company or because the kids have been guzzling it, only to find myself with slowly turning cider because they've hit their saturation point.
Unfortunately, I think vinegar is made from hard cider. The alcohol turns to vinegar, with the assist of a vinegar mother. I don't think you can use soft cider, but I may be wrong.
Perhaps you can use it in baking or cooking. It might be nice to add some to a muffin batter. Do you eat meat? I have a few chicken and pork recipes that use apple juice in the marinade - I bet cider would be just as nice.