Wash the jars and lids in hot soapy water or run them through the dishwasher, then let them dry fully. Starting with clean jars keeps the finished jam tidy and ready to fill as soon as the cooking is done.

Set out a large heavy-bottomed nonreactive saucepan or a Dutch oven that holds at least 4 quarts. The pot needs extra room because the jam will rise and bubble actively once it reaches a full boil.

Trim off both ends of the rhubarb, including any leafy parts, and discard them, then wash the stalks and cut them into 1/2 to 1 inch pieces. If any stalks are thick, split them through the center first so the pieces soften and break down evenly.

Split the vanilla bean and scrape out the seeds, keeping both the seeds and the pod. Having them ready now makes it easier to add the pod early for infusion and the seeds later for a cleaner vanilla finish.

Combine the rhubarb, orange juice, orange zest, sugar, and vanilla pod in the pot, then stir until the fruit is well coated. As the sugar sits on the cut rhubarb, it begins pulling out juices that help the batch start cooking evenly.

Set the pot over medium-high heat and bring the mixture to a simmer, then raise the heat to high and cook until it boils hard, stirring often. The fruit will first look chunky and wet, then quickly loosen into a vivid pink mixture with fast, noisy bubbles.

Spoon off any foam that gathers on the surface and discard it. Removing the foam helps the jam look clearer and more jewel-like in the jar.

Keep the jam bubbling while stirring and adjusting the heat as needed, and press the rhubarb with a spoon to break it down. The sharp grassy scent will mellow into a sweeter, deeper aroma as the fruit collapses and the mixture thickens.

Cook until a thermometer reads between 220 to 222°F (104 to 105°C), which usually takes about 15 to 20 minutes, then take the pot off the heat and briefly set the bottom in shallow water to stop the cooking. At this stage the bubbles look thicker, the spoon leaves a short trail, and the jam coats the back of the spoon with a glossy sheen.

Stir in the scraped vanilla seeds. This keeps the tiny specks fragrant and visible instead of cooking them flat for the entire boil.

Let the jam cool for 10 minutes, stirring now and then, then remove the vanilla pod. That short rest helps the fruit settle into the syrup so the texture stays more even in the jars.

Spoon the jam into the clean jars, leaving about 1/2 inch headspace, add the lids, cool to room temperature, then chill in the refrigerator overnight to finish setting before freezing. The headspace matters because the jam expands as it freezes, and the finished texture firms up noticeably as it gets fully cold.