For the soup, steam rhubarb in chunks in butter until just tender, shaking occasionally with lid closed. Stir through remaining ingredients. Pour into rhubarb while stirring carefully. Bring briefly to a boil. Cover and allow to cool.
For the vanilla curd, slit vanilla bean lengthwise, scrape out seeds.
Stir through all ingredients. Cover and set aside to cool.
Divide rhubarb soup evenly into glasses. Fill vanilla curd into a piping bag with a serrated nozzle, squirt on top.