Place the red wine, caster sugar and vermouth into a sauce pan.
Cut the orange in half. Add the juice of one half into the sauce pan and bring the mixture to the boil slowly. Once it's reached a boil reduce the heat to a simmer.
Slice the remaining half of the orange and the whole line and add the slices to the saucepan along with the whole spices. Let the mixture simmer for 15 minutes.
Pour the mulled vermouth into mugs or heatproof glasses and enjoy while still hot.