Combine sugar, water, and vanilla bean in a small-to-medium saucepan, stirring occasionally until sugar dissolves. Bring to a boil. Cool and remove vanilla bean (to hasten cooling, place the syrup in a bowl, and place the bowl in another bowl filled with a mixture of ice and water). Combine about 1 cup sugar syrup with the citrus juice. Taste and add more syrup if necessary. Stir in the zest. Refrigerate until cool if necessary, then churn in an ice cream maker according to the manufacturer's instructions. Or, if you don't have an ice cream maker, place the bowl in the freezer and stir it every thirty minutes until it achieves an Italian-ice-like consistency.