Wash plums; cut into small pieces, and remove pits. Cook plums slowly in a small amount of simmering water until they are softened. Crush fruit with a potato masher.

Measure the crushed plums into a large saucepan. Add lemon juice and powdered pectin; stir well. Split the vanilla bean lengthwise with a small, sharp knife, and cut into several pieces. Add vanilla bean segments and cinnamon sticks to plum mixture; place pan over high heat. Stirring constantly, bring mixture to a full rolling boil. Add sugar; continue stirring, and heat again to a full, rolling boil. Boil hard for 1 minute. Remove pan from heat; skim foam from surface. Remove and discard cinnamon sticks.

Cool jam for about 5 minutes, stirring occasionally. Ladle hot jam into hot, sterilized, half-pint jars, evenly distributing fruit among jars, leaving 1/4-inch headspace. Release air bubbles with a thin spatula. Wipe jar rims with a clean, damp cloth. Adjust two-piece canning lids. Process in a boiling-water bath for 10 minutes. Cool, undisturbed, for at least 12 hours. After 12 to 24 hours, check seals (and refrigerate any jars that haven't sealed properly). Remove screw bands, wipe jars and lids, label, and store in a cool, dry, dark place for up to 1 year.

Makes 9 to 10 half-pints.