Make sure the pears can fit snugly in the pot you will use - it's fine if you need to put the tops to one side, but you don't want too much space around them. Set the pears to one side.
Place the water, sugar, vanilla pod, star anise and strips of lemon and orange zest (you can use more than one of each if you like) in the pot. Place over a medium heat and bring to a simmer, stirring now and then to dissolve the sugar.
Meanwhile, peel the pears, being careful to leave the stems attached. Remove the core from the base with either a corer or knife, carefully scooping out without damaging the pear itself. Then, peel over the bottom once to make the base slightly flat so that it stands easily.
Once the poaching liquid is simmering, carefully add the pears into the liquid, letting them be as well covered as you can (they will likely float a little). Simmer the pears in the liquid around 20 - 25 minutes, turning now and then to make sure all sides are submerged at some point. You can test the pear with a skewer or thin sharp knife to make sure they are tender, if you like, then remove the pot from the heat. Let the pears cool in the poaching liquid.
Once the pears have cooled sufficiently, transfer the pears to an airtight container and add the poaching liquid. Cover and store in the fridge for 3-5 days until ready to use.