Vinegar softens the bite of raw onions

The secret to really crispy skin has everything to do with rendering the fat that lies directly under it. Once that fat has removed itself from the skin, a little high heat will crisp it up perfectly. If I am creating a roast chicken, I normally remove it from the packaging and allow it to sit uncovered in the fridge for at least overnight, longer if I have the time. Then I start the chicken low to render the fat, you have to cook it at a minimum of 178F (80C) to remove the most fat. Once the fat renders out, which is going to vary depending on the temp you start the chicken at, you crank the temp to the 450 to 500F range and baste regularly.

To maximize the skin that will crisp you want to make sure that your chicken isn't sitting inside the fat while it cooks. I normally use a inset rack to keep the chicken above the fat but you don't have to. The other component to crispy skin is cooking the chicken uncovered.

From a safety point of view, assuming you are keeping the vinegar in the refrigerator while soaking...

I would make the conservative assumption that the water and nutrients from the onions is diluting the acidity of the vinegar, and making it a more hospital environment for pathogens. Therefore, I would think of the vinegar as a fully perishable item, with a lifetime in the refrigerator of four or five days.

How many batches of onions could you soak in that time?

Of course, from the efficacy point of view, each batch of onions will flavor the vinegar more, and dilute it more--so at some point, you won't get the results you are looking for, whether that is a basic "velveting" of the onions from a quick soak, or a more pickling like change from a longer soak. You will have to decide when the results are no longer good enough.

The thing is--vinegar is very inexpensive, at least in the US. Is it worth reusing it, when you will get less consistent results?