Put the redcurrants in a saucepan over a medium heat and add a generous splash of boiling water. Heat until soft.
Meanwhile, heat the rhubarb, vanilla seeds and sugar in a separate saucepan over a low heat until the sugar has dissolved and the rhubarb has softened.
Strain the redcurrants through a fine sieve into the rhubarb pan. Stir in the lemon juice.
Bring the mixture to the boil, then continue to heat until its temperature reaches 105C (use a sugar thermometer for this; it will take about 20 minutes). Scoop away any scum that rises to the surface using a metal spoon. (CAUTION: hot sugar can be dangerous. Keep your eye on the pot at all times.)
Remove the pan from the heat and set aside to cool for 20 minutes.
Pour the jam into sterilised jars and seal. Set aside until completely cooled.
This jam will keep for up to 6 months.