1. Combine rhubarb, sugar, vanilla bean seeds, and 1/2 cup water in a large saucepan over medium-high heat. Bring to a boil; reduce heat to medium and simmer until rhubarb has broken down and mixture is thick and syrupy, 12 to 15 minutes.

2. Transfer rhubarb mixture to the jar of a blender along with raspberries and lemon juice; blend until smooth. Set a fine mesh sieve over a large container. Strain rhubarb mixture into container; cover and transfer to refrigerator until chilled.

3. Freeze rhubarb mixture in an ice cream maker according to manufacturer's instructions. Keep frozen, in an airtight container, up to 1 week.