Place the sugar and the water in a medium saucepan; bring to a boil, stirring occasionally, until sugar is dissolved. Boil 2 minutes; Remove from heat. Cut vanilla bean in half lengthwise, and scrape seeds from vanilla bean. Place seeds and vanilla bean pod into syrup. Cool to room temperature. Remove pod.
Place mascarpone in a medium bowl, and slowly whisk in vanilla syrup, trying to work out any lumps before adding too much syrup. Add lemon zest, lemon juice, and lemon extract. Refrigerate mixture for 2 hours, or overnight. Pour into an ice cream maker, and freeze according to manufacturer’s directions. Transfer sherbet to a 2-quart container and into the freezer. Sherbet will be soft.