Mix the rhubarb, lime juice, vanilla seeds and preserving sugar in a wide-bottomed pan, bring to the boil, then boil fast for approx. 2 mins. while stirring.
Remove the vanilla pod, ladle the jam into the clean, pre-warmed jars while still boiling hot, fill the jars to just below the rim, seal immediately. Briefly upend the screw-top jars, leave to cool on an insulating surface.