Cut rhubarb into 1/2-inch pieces and macerate in 1/2 cup sugar for 1 hour. Meanwhile, puree strawberries with remaining 1/4 cup sugar. In a saucepan, cook rhubarb and orange juice, covered, until mushy. Remove from heat and stir in strawberry puree.
For the syrup: Bring ingredients to a boil. Set aside to cool.
For the anglaise: In a saucepan, bring the milk cream and vanilla bean to a scald. In the meantime, in a bowl, whisk together the yolks and sugar. While whisking the yolks, drizzle in the hot milk. Return to the pan and cook, stirring until thick enough to coat the back of a spoon. Strain and chill in an ice bath. Add the lemon syrup to the buttermilk. Mix into creme anglaise.
Drizzle the anglaise mixture on top of the strawberry rhubarb nothing. (Soup may be served hot or cold.)