Prepare a small boiling water bath canner and 2 half pint jars. Place the chopped rhubarb in a low, wide non-reactive pan. Whisk the pectin and vanilla bean seeds into the sugar and add it to the fruit. Drop the split vanilla bean into the pan and add the lemon juice. Stir well and let it sit until the sugar looks damp. Set the pan on the stove over high heat and bring to a boil. If the sugar begins to caramelize, reduce the heat. Cook, stirring regularly, until the rhubarb breaks down and the liquid looks thick and jammy. Remove the pan from the heat and divide the jam between the two prepared jars (depending on how much water the rhubarb contained, you may have a couple tsp. leftover. I recommend stirring the leftover into some plain yogurt). Wipe the rims, apply the lids and rings, and process in a boiling water bath canner for 10 minutes. When the time is up, remove the jars from canner and set them on a folded kitchen towel to cool and seal.