Put the raspberries, sugar and vinegar in a medium saucepan. Simmer over a low heat for 10 minutes until the sugar has dissolved, the raspberries have broken down and the liquid is syrupy. You may need to mash the raspberries with a fork.
Strain through a sieve over a large saucepan. Place the saucepan on a high heat, add 350ml cold water and bring to a boil. Continue to boil for 3-4 minutes, until slightly thickened.
Pour into a sterilised bottle and seal. To serve, dilute to taste with your liquid of choice or make a raspberry float with lemonade and a scoop of good quality vanilla ice cream. The cordial will keep in the fridge for a week.