Remove the rind from the grapefruits. Thickly slice the fruit and then cut or break apart into pieces. Discard any seeds.

Puree the fruit in a food processor or blender, and then pour it into a heavy bottomed pot, along with the sugar.

Cut the vanilla beans in half and then make a slit down the length of each piece. Pry the beans open with the tip of your knife and scrape out the seeds, adding them to the pot with the grapefruit. Throw the pods right in, too.

Bring the pot up to a boil, then turn down and boil for about 40-50 minutes. You won't have to tend it much at first, but toward the end you'll need to keep an eye on it and stir often to prevent scorching. It will be greatly reduced, darker, and thicker at about the 40 minute mark. The longer you boil it the thicker the jam will be, but I found about 45 minutes was sufficient. Remove the vanilla bean pods and discard.

Pour the hot jam into jam jars and let cool before covering and refrigerating. Use within 2 months, or freeze.