Makes 1 qt.
1. Place nectarines, sugar, water, and vanilla bean in a saucepan over medium, and cook, stirring occasionally, until the nectarines are soft, 3 to 5 minutes. Remove pan from heat, and let nectarines cool to room temperature. Remove vanilla bean, and discard, or rinse and reserve for another use.
2. Process nectarine mixture, cream, lemon zest, and lemon juice in a blender until almost smooth. (There should be some pieces of nectarine throughout the ice cream.) Chill overnight.
3. Place chilled mixture in a freezer container of a 1-quart electric ice-cream maker, and freeze according to the manufacturer's instructions. Transfer to an airtight container, and freeze until firm but still scoopable, at least 4 hours.