Step 1Heat the oven to 300 degrees. Clean the rhubarb and trim off the ends. Cut the rhubarb into roughly 2-inch lengths and arrange in a baking dish in as close to a single layer as possible. Grate the zest from half of one of the oranges, sprinkle over the rhubarb and then drizzle with the juice of the whole orange. Split the vanilla beans lengthwise and scrape the seeds over the rhubarb. Sprinkle with light brown sugar.
Step 2Toss to mix well and bake until the rhubarb is quite tender and fragrant and the sugar and orange juice have melted together, 15 to 45 minutes depending on the size and toughness of the rhubarb. Serve at room temperature, dividing the rhubarb and sauce evenly among the servings.