If you have access to a food processor, mince the cranberries and rhubarb to a fine pulp. Otherwise use a pastry blender or food chopper and coarsely chop the cranberries and rhubarb. Leave some cranberries whole, if desired.
Put cranberries and rhubarb in a large glass jar.
Add sugar, vodka and water. Stir. Cover tightly and set in a dark place at room temperature for at least 3-6 months.
Before serving, strain cordial. Strain through double thickness of cheesecloth. Discard fruit.
Store cordial in a jar or decorative bottle with a tight fitting top.