Position a rack in the center of the oven and preheat to 200°F. In a small bowl, rub the vanilla seeds into the raw sugar until incorporated. Pour in the Shrubb and stir to combine. Using a silicone spatula, spread the sugar mixture in an even layer on a parchment-lined baking sheet. Bake with the oven door slightly ajar until dried, about 30 minutes. Remove from the oven, and when cool enough to handle, use your hands to crumble; set aside.
Oil an 8-by-8-inch baking pan and line with parchment. In a small bowl, whisk together the pectin and ¼ cup of the sugar; set aside.
To a medium pot set over medium-high heat, add the vanilla pod, rhubarb, and 1 cup of the sugar and cook until the rhubarb is soft, 8–10 minutes; set aside to cool, about 45 minutes.
Discard the vanilla pod and transfer the rhubarb mixture to a blender (do not clean the pot). Add the corn syrup and strawberries and purée, then pour back into the pot and turn the heat to high. When the liquid boils, whisk in the pectin mixture and cook, stirring continuously, until dissolved, 1–2 minutes. Whisk in 1 cup of the sugar and cook, stirring continuously, until dissolved, 1–2 minutes more. Repeat with the remaining sugar, then continue cooking until the mixture has thickened and a thermometer reads 175°F, about 5 minutes more. Stir in the lemon juice, then pour into the prepared pan. Let stand at room temperature until set, about 4 hours.
To serve, cut the pâte de fruit into 1-inch squares and sprinkle with the reserved Shrubb sugar. (Pâte de fruit will keep in an airtight container at room temperature for up to 1 month.)