Pack the figs into a warm, dry jar with the cut sides facing out. Pack the centre of the jar tightly and put two halves, cut side uppermost, in the top of the jar.
Put the sugar, measurement water and vanilla pod into a saucepan. Slowly bring to the boil and heat, stirring continuously, until the sugar has completely dissolved. Boil for 1 minute, then remove from the heat.
Lift the vanilla pod out of the syrup and, using a small knife, scrape out the black seeds into the syrup and stir in the citric acid. Tuck the vanilla pod down the side of the jar.
Pour the syrup over the figs to cover completely and come almost to the brim of the jar. Top up with boiling water if needed and seal the jar. Stand the jar on a baking sheet lined with several sheets of folded newspaper and bake in a preheated oven, 150°C (300°F), Gas Mark 2, for 40 minutes until the syrup has turned a delicate pink and the figs are just beginning to rise in the jar.
Using oven gloves, transfer the jar to a wooden board, secure the lid and leave to cool completely. When cold, check the jar seals, label and store in a cool, dark place.