While most table vinegar's have an acid content of around 5% to 8%, a more concentrated solution of 10% to 15% can be achieved simply by exposing vinegar to the air and allowing it's water content to reduce by evaporation and so increase it's acidity level. The higher the acidity level the more effective your solution will be at killing weeds.
At high strength this organic weed killer will kill the leaves of any plant it comes into contact with but will not kill its roots, so it's best used on young weeds which lack enough energy to re-emerge. Repeated spraying destroys any more established weeds.
You can make your own organic weedkiller by mixing 150 mls of lemon juice and 1 liter of strong vinegar.