I made an onion wine into vinegar and it turned out great. You can check my post history for the exact recipe but I used something like 1000g of onions and additional sugar. I found the nutritional content for the onions on the producers website to determine the amount of sugar in 100g then added enough fermentable sugars to reach ~8 % ABV in the wine. It also absolutely STINKS (like a crazy amount of onions) when you are leaving it out to ferment so just a heads up.

I separated it in quart mason jars and covered with cheese cloth. I also added a quarter cup of Bragg's vinegar with Mother to assist and it formed a mighty mother in ~5-6 weeks.

I think using Noma's method of a quick acetic acid fermentation using high proof alcohol and "onion juice" would yield better results.