In a medium saucepan over medium heat, combine 2 cups of water with the allulose. Bring to a boil and cook 5 minutes. Let cool 15 minutes.
In a blender or food processor, puree the berries with remaining 2 tbsp of water and the lemon juice.
Strain half of the mixture through a fine mesh sieve, pressing on the solids to release as much juice as possible. Discard the seeds and repeat with the remaining puree. You should end up with about 1 1/2 cups.
Combine the raspberry puree and the water/sweetener mixture and refrigerate until well chilled, about 2 hours. Taste and adjust sweeteness as desired.
Whisk in the vodka, if using. Pour into an ice cream maker and churn according to manufacturer's directions. Once the sorbet reaches the consistency of soft serve ice cream, transfer to an airtight container and freeze until firm.