Combine the chopped rhubarb, water, and scraped vanilla bean seeds and pods in a large, heavy-bottomed saucepan. Bring to a simmer over medium heat.

Cook, stirring occasionally, until the rhubarb has broken down completely and the mixture is pulpy, about 15-20 minutes. Mash any larger pieces with the back of a spoon.

Strain the rhubarb pulp through a jelly bag or a sieve lined with cheesecloth set over a bowl. Allow the juice to drip through naturally for at least 4 hours, or preferably overnight. Do not press the pulp, as this can make the jelly cloudy.

Discard the pulp and the vanilla pods. Measure the reserved jelly juice. You should have around 750ml - 1 litre. For every 500ml of juice, you will need approximately 450g of sugar. Adjust sugar quantity based on your yield.

Pour the jelly juice and the adjusted amount of sugar into a clean saucepan. Add the lemon juice. Heat gently, stirring to dissolve the sugar completely.

Once the sugar is dissolved, increase the heat to high and bring the mixture to a rolling boil. Boil rapidly for about 10-15 minutes, or until the jelly reaches setting point. Test by dropping a little onto a chilled saucer; it should wrinkle when pushed with a finger.

Remove any scum from the surface. Skim off any remaining scum, then carefully ladle the hot jelly into sterilized jars, filling them to the brim. Seal immediately with sterilized lids.