Makes 4 cups
Bring sugar and 1 1/2 cups water to a boil in a saucepan. Reduce heat to medium and cook, stirring often with a wooden spoon, until sugar dissolves, about 10 minutes.
Meanwhile, place cranberries in a food processor; process to a smooth puree, about 2 minutes, scraping down sides of bowl several times. Add cranberries to hot syrup and stir well. Cook 10 minutes more.
Pour cranberry syrup into a bowl set in an ice water bath. When cooled, transfer to refrigerator until chilled. Add buttermilk and vanilla to cranberry syrup. Transfer to an ice cream maker and freeze according to manufacturer's instructions. Store frozen in an airtight container for up to a week.