Chop the rhubarb into small cubes, enough for 2 cups.
In a saucepan, stir together the rhubarb and the sugar over low heat. Once it starts to to bubble, cook for about 10 minutes until the rhubarb is broken down and the juices have slightly thickened, mashing with the back of your spoon (exact timing depends on the size of the rhubarb and heat level).
Remove from the heat and stir in the lemon juice and vanilla. Transfer to a ball jar and chill for at least an hour, which will thicken the juices even more. Store refrigerated for a few weeks. Serve over waffles, pancakes, or swirled into oatmeal, yogurt, or ice cream.