Add chopped rhubarb, sugar, and water to a medium saucepan. Cook over medium heat for 10–15 minutes, stirring now and then, until the rhubarb is soft and thick like a chunky sauce.

Take it off the heat, stir in the vanilla, and let it cool. If it looks too watery, just simmer a bit longer.

In a bowl, mix mascarpone, powdered sugar, and vanilla until smooth. Then gently fold in the whipped topping using a spatula. Stop once it's creamy and fluffy—don’t overmix.

Pour orange juice or rhubarb syrup into a shallow bowl. Dip each ladyfinger quickly—about one second per side—and place in a single layer in an 8x8-inch dish. They should be moist but not soggy.

Spread half of the mascarpone mixture over the ladyfingers. Spoon half of the rhubarb mix on top. Repeat with another layer of dipped ladyfingers, the rest of the mascarpone, and finish with the last of the rhubarb.

Cover the dish with plastic wrap and refrigerate for at least 4 hours, or overnight. This helps everything set and makes it easier to slice and serve.