Combine the chopped rhubarb, granulated sugar, and water in a medium heavy-bottomed saucepan.

Place the saucepan over medium heat. Stir gently until the sugar begins to dissolve.

Bring the mixture to a simmer. Once simmering, reduce the heat to low, cover the saucepan, and cook for 15-20 minutes, stirring occasionally.

The rhubarb should break down completely and the sauce should thicken slightly. If you prefer a completely smooth sauce, you can mash the remaining larger pieces with a spoon or use an immersion blender for a few quick pulses.

Remove the saucepan from the heat and stir in the vanilla extract and lemon juice.

Allow the sauce to cool to room temperature, which will allow it to thicken further. Once cooled, store in an airtight container in the refrigerator for up to two weeks.