CanMan, there are lots of reasons why old-time canning methods and recipes might not be safe. But the way you phrased it could be misleading.
A vinegar that's 18% acidic is more than three-times as acidic as modern store-bought stuff. So, presuming the old-time recipe cut back on the amount of vinegar, then modern stuff could, indeed, be too weak.
However, that wasn't usually the case. Back in the day they preferred a sharper, more sour taste then we do today. And so, in terms of quantitites, there was much more vinegar in the pickling brine then there is in modern recipes. In addition, they were less likely to cut it with water.
So much so that we would likely find their pickles to be nearly inedible.
Here, for instance, is Mary Randolph's 1823 recipe:
"To Pickle Cucumbers:
Gather them full grown, but quite young--take off the green rind, and slice them tolerably thick; put a layer in a deep dish, strew over it some chopped onion and salt; do this until they are all in; sprinkle salt on the top, let them stand six hours, put them in a colander---when all the liquor has runoff, put them in a pot, strew a little cayenne pepper over each layer, and cover them with strong cold vinegar; when the pot is full, pour on some sweet oil and tie it up close; at the end of a fortnight, pour off the first vinegar and put in fresh"
Mary Randolph's vinegar would likely have tested at 20%.
Once you translate that into modern English, the Ball Blue Book has a recipe that is virtually the same. Only significant difference is that the BBB calls for 2 cups of modern (5%) vinegar mixed with two gallons of water. Which makes for a pretty thin brine, compared to Ms Randolph's. And the vinegar isn't changed after two weeks.
Just for the record, that works out at .125% acidity, and Ball is counting on the salt to perform a lactic acid fermentation. However, if you followed Randolph's recipe, using modern vinegar, you would have 5% acidity working along with the salt for the same lactic acid fermentation. And you'd be refreshing the acid as well.
The general rule of thumb, in modern canning, is to mix up to one cup of water with one cup of vinegar. Which means you can dilute it to about 2.5% and still have what every food scientist in the world considers a safe acid level. That being the case, there is nothing unsafe about the Randolph recipe. Nor many---perhaps most---old time recipes.
The trick is understanding the processes involved, rather than slavishly following the mandates of a government agency that has proven itself inept time after time.