Put the raspberries in a pan with the sugar and vinegar. Mash over a low heat for 10 mins until smooth and syrupy. Rub through a sieve into a clean pan.
Tip the seeds from the sieve into a bowl and stir in 300ml water, then sieve again to remove the last of the pulp from seeds. Pour the liquid into the pan with the sieved pulp, stir well and boil for 1 min. Pour into small sterilised bottles and seal. The cordial will keep unopened for a few months. Once opened, store in the fridge.