Cut the peel from the grapefruit leaving most (but not all) of the white pith on the fruit. Slice the peel into thin strips.
Place peel in a pot and cover with water. Bring to a boil. Once it starts to boil, remove from the heat and cover. Let it sit for about 20 minutes. Drain and repeat.
Cut off all of the white pith from the fruit. Working over a bowl or a large pot, separate the sections of the fruit, peeling off the membrane and removing the seeds. Squeeze as much juice from the membranes as you can. (Discard all seeds).
Add the peel and sectioned fruit (with juice) to a large pot. Add the water and sugar. Bring to a boil and stir to combine. Add pectin and stir until completely dissolved.
Lower the heat to a low simmer. Cook until the marmalade is thick and translucent.
Add the vanilla bean seeds and simmer another five minutes. Remove from the heat and stir in the lemon juice.
Ladle into jars and process in a hot water bath for 10 minutes. Let rest on a dry towel until cool and the lids have popped. Any unpopped jars can be stored in the fridge or re-processed.