The Jackson County Master Gardeners’ recipe for making your own vinegar based weed killer:

• 1 gallon 10 per cent vinegar
• 1 cup of salt
• 2 tsp Dawn dish soap or vegetable oil (to make it adhere to the plant)

Also, read some pros and cons of using vinegar (Acetic Acid) as a herbicide here:
https://www.thespruce.com/vinegar-acetic-acid-as-a-herbicide-1402744