Wash the rhubarb stalks well under cool water and dry gently with a towel. Trim the ends and chop the rhubarb into small 1-inch pieces.
Add the chopped rhubarb to a medium saucepan. Pour in the sugar, water, vanilla extract, and lemon juice. Stir gently until the rhubarb is coated with sugar.
Place the saucepan over medium heat. Cook the mixture slowly while stirring every few minutes to prevent sticking.
After about 5 minutes, the rhubarb will begin softening and releasing liquid. Continue cooking for another 8 to 10 minutes until the sauce becomes thick, glossy, and soft.
Use a spoon or potato masher to mash the sauce to your desired texture. Leave some chunks for a rustic texture or mash more for a smoother sauce.
Remove the saucepan from the heat and allow the sauce to cool slightly. The sauce will continue thickening as it cools.
Serve warm over pancakes, waffles, cheesecake, oatmeal, yogurt, or ice cream. Store leftovers in the refrigerator.