Combine the rhubarb, berries, the split vanilla bean and a cup of the sugar in a large, non-reactive pot. Stir until the sugar has drawn some liquid out of the berries and then turn the heat onto medium low.
Cook over medium low heat, stirring regularly, until the pieces of rhubarb and strawberries have broken down. At this point, reduce the heat even further and let the butter simmer over very low heat.
When the butter has reduced to about half its original volume, taste it. Adjust the sugar to taste, adding up to another half cup. Cook until the sugar has dissolved into the butter. Remove the butter from the heat. Fish the vanilla bean pieces out and put them aside.
Fill your prepared jars, wipe the rims, apply lids and screw on bands. Process filled jars in a boiling water bath for 15 minutes (starting time when water returns to a boil).When time is up, remove jars from pot and let them cool on a towel-lined counter top. When jars are cool enough to handle, remove rings and check the seals.
This butter will keep 6 to 9 months if stored in a cool, dark place. It is good to eat right away, though.