Combine the plums, sugar, and water in a heavy bottomed sauce pan. Crush the fruit with a potato masher then add the vanilla bean to the pot. Bring the mixture to a boil over medium-high heat then reduce the heat to medium and simmer for 30 minutes, stirring occasionally until the liquid has reduced and the mixture has thickened.
Place a fine mesh strainer over a large bowl and pour the plum mixture into the strainer. Press on the solids with a spatula to extract all of the syrup then discard the solids. Allow the syrup to cool then store in the refrigerator for up to a week.