In a large skillet add water and plums, cook on low heat until plums start to reduce. Cook for 15-20 minutes, stirring occasionally. Use the back of a fork to smash plums if you want them a little more mashed.
On medium-low heat, and add sugar and lemon juice. Stir continuously until all ingredients are well incorporated.
Bring to a boil, stirring continuously. Add vanilla. Cook for 5 minutes. If jam has any froth on top, remove with a spoon, and discard.
You can eat the jam right away, cool and keep in the fridge for later, or store long-term in sterilized jars sealed with lids.