I'm concerned now about killing my grass not just weeds. I have a new lawn, laid just a few weeks ago. Should I take any precautions when appling the vinigar solution to a new lawn.

If you can find 20% vinegar, use it full strength.

Some grasses are not affected at all by vinegar. Spray a small spot and see what happens. Give it 3 days to respond.

If you have weeds, though, then you are not caring for the lawn properly. Watering too frequently and improper mowing are the usual offenders. If you are doing both of those right, then you should fertilizer more heavily and frequently (organic of course).