Place the quince pieces and the lemons in a large stock pot. Pour over the water until all the fruit is covered.
Bring the water to a boil and simmer the fruit for around an hour, until the quince is very soft. Allow it to cool to room temperature in the pot.
Once cooled, pour the fruit and the liquid into a cheesecloth or muslin cloth, suspended over a large bowl.
Let the quince drain for at least 8 hours to collect all the liquid. It's easiest to leave it to drain overnight.
Once the liquid has been drained, measure it out in cups and add 1 cup of sugar per 1 cup of liquid into a saucepan.
Scrape the seeds out of the vanilla pods and and add them to the liquid. Add the empty pods in as well.
Bring it to a boil and let it simmer for around 15-20 minutes until it is a deep red colour and at a jelly setting stage. Stay close to the stove stir it regularly so it doesn’t boil over. Any scum building up can be scooped off.
To test that the jelly is ready to be poured into jars, take a teaspoons worth and drop it on a plate. Let it cool for a minute.
Once cooled, run a finger through it and see that it leaves a line, without the jelly running back to the join in the middle. If it’s too runny, keep it simmering a while longer, otherwise it won’t set.
Once it is at the jelly setting stage, remove the empty vanilla pods. Leave the jelly to cool for a bit, then the jelly can be poured hot and clean preserving jars.
Seal the jars and simmer them in a boiling water bath for 8 minutes to suction-seal them for long-term storage.