1. Bring sugar, 1 cup water, and salt to a boil in saucepan; cook 1 minute, or until sugar dissolves. Remove from heat, and scrape seeds from vanilla bean into syrup (don’t worry if they stick together), then add vanilla bean to syrup. Cool; remove vanilla bean from syrup, and reserve for another use.
2. Blend avocados 3 minutes in blender or food processor until very smooth. Add half of lime juice, and blend until combined. Add remaining lime juice, and then vanilla syrup; blend until smooth.
3. Chill, then churn in ice cream maker according to manufacturer’s directions. Transfer to 1-quart container, and freeze.