pickled shallots

Rick of Rick's Picks here. Agree with the above on a 1:1 ratio as a baseline for a simple vinegar/water brine, with a bit fo kosher salt added, but only if acidity of the vinegar is 5% or higher. However, red wine vinegar is intense and will have a big impact on the flavor profile of your pickled shallots... to maximize flexibility of use of the finished product, you might want to try some with white 5% vinegar, which is much more flavor-neutral.

ETA: pH goes DOWN with increasing acidity, so that really should say "the maximum pH for safety is 4.6" - you want it at 4.6 or LOWER for water-bath canning. A 1:1 ratio of vinegar to water will usually be adequate if your vinegar is at 5% acidity or higher. If you have a pressure canner, then the pH isn't relevant, but will lose much of the "bite" from the long high-temp cooking.

The minimum pH for safety is 4.6 - the best way to ensure you meet this level is to get a litmus test kit from a hardware store and test your brine before you pour it over your shallots.