It has to do with the acidic content of the vinegar. Distilled white vinegar is 10%.
Here are 2 tonics from Jerry BAker -- both have the warning to not get them on plants you want to keep and this first one warns not to pour it on soil you plan to garden one.
Wonderful Weed Killer
1 gal vinegar
1 cup table salt
1 tbsp diswashing liquid
Mix ingredients in a bucket until salt desolves. Pour solution along cracks to kill weeks between bricks or stones in walkways.
All Purpose Weed Killer
5tbsp of vinegar
2 tbsp of table salt
1 qt water
Bring water to a boil then add vinegar and salt. Wile mixture is still hot, pour it directly on the weeds, then wave good-bye.
Here's one for the really stubborn weeds.
Wild Weed Wipeout Tonic
1 tbsp gin
1 tbsp white vinegar
1 tbsp baby shampoo
1 qt of warm water
Mix these ingredients in a bucket, then pour tonic into a hand held sprayer. Drench each week to the point of run-offm taking care not to get any spray on the surrounding plants. For stubborn weeds, use apple cider vinegar instead of white vinegar.
The distilled white vinegar I bought two gallons of at Sam’s Saturday was 5%. Where can you find 10%?..........