In a large saucepan over medium heat, combine the cranberries and water. Bring to a boil and cook until the cranberries have popped.
Pour into a sieve set over a bowl and press on the solids to release as much juice as possible. Stir in the sweetener and orange extract and chill 1 hour.
For the cocktails, add 2 tbsp cranberry juice and one tablespoon of vodka to each champagne glass. Top up with chilled champagne.
Garnish with a rosemary sprig and fresh cranberries, if desired.