Put unthawed raspberries in a saucepan. Add sugar. Cook over medium heat, stirring once in a while, for 20-25 minutes, until berries are thawed and sugar dissolved.
With potato masher, mash berries and syrup thoroughly. Pour berry mixture into a wire strainer, making sure to extract all the juice by using the back of a wooden spoon to push it through. Discard pulp.
Pour raspberry syrup into a pitcher. Squeeze lemons and strain juice. Add lemon juice to raspberry syrup. Boil water and add to raspberry syrup.
Cool raspberry cordial to room temperature, then cool in fridge. When completely cooled add to the vodka and refridgerate again. When cordial is ready to serve, stir well.