Take the skin off grapefruits and chop the meat into chunks. (Make sure to take off as much white, soft part of the skin as you can.) Take out the seeds.
Combine grapefruit meat, vanilla, grapefruit juice and sugar in a medium sauce pot, mix well, cover and bring it to a boil over medium-high heat.
Turn the heat down to medium-low, take off the lid, and simmer for 25-30 minutes. Stir occasionally.
For syrup: run the cooked grapefruit mixture through a fine strainer. Press the cooked meat down in a strainer, using a spoon, to get remaining juice out of it. Cool and store in the refrigerator, in an air-tight container or jar.
For sauce: Once the grapefruit mixture is cooked, transfer everything into a blender. Open that little flap on the lid to help hot steam escape. Blend well, until the mixture in completely smooth. Cool and store in the refrigerator, in an air-tight container or jar.