Vinegar makes an effective organic weed and grass killer, according to SFGate. Household vinegar mixed at 5 percent is good for weeds while pickling vinegar mixed at 20 percent is more effective for grass. Vinegar is non-selective, meaning it will have the potential to kill everything organic it touches.
The more acetic acid the vinegar has, the more effective it will be. Also, using the vinegar on a sunny day can help burn the acid into both grass and weeds more than it can on a cloudy day. Vinegar has the ability to change the soil pH if used repeatedly, so care should be taken for plants needing alkaline soil.