Wash and drain the blueberries. Put them in saucepan, add the sugar and lemon juice.
Bring to the boil gently, stirring with a wooden spoon until the sugar has dissolved.
Cook for about 10-15 minutes, until it’s thick and sticky. (You could test the jam, if you like, by putting a drop on a chilled plate then placing it in the fridge for 30 seconds. Then run your finger through it - if it’s tacky and wrinkled it’s good; if not, cook for another couple of minutes and retest.)
Mix in the vanilla and a drop of lemon juice.
Transfer the jam into a sterilised jar and seal with the lid.
Keep it in the fridge.