When I make pickled onions, I peel them and put them in a bowl in layers covered with lots of cooking salt. This draws the excess water out of the onions. I leave them for about a day, shaking the bowl occasionally. Drain, quick rinse and dry then put in jars with pickling vinegar (Sarsons ready spiced). Sarsons do a great booklet with some really interesting recipes.
I reckon re-using the vinegar will probably work once or twice but it will get diluted slightly with the onion juice and lose its preserving qualities - it needs to be about 6% acetic acid. Hope this helps. Cheers, Andy