Place cinnamon, star anise, cardamom and cloves in a large saucepan or cast iron pot. Dry roast spices over medium heat for 1 - 2 minutes until fragrant.
Remove from the heat, allow to cool slightly and then add wine, sugar, orange slices and vanilla.
Return to the stove and bring to a boil, then reduce heat to a medium boil, letting the spiced wine bubble away until it turns into a thick syrup.
Remove from the heat, cover with a lid, and leave to infuse for 30 minutes so the flavours can develop.
Strain the mulled wine syrup and transfer it to a sterilized air-tight bottle. Will keep in the fridge for up to one month.