Split vanilla bean; scrape seeds into a saucepan; place bean in pan. Add sugar, water, and lemon slices. Bring to a boil over medium heat. Reduce heat to low, and simmer, stirring occasionally, 10 minutes. Remove from heat; let stand 5 minutes. Pour through a fine wire-mesh strainer into a bowl; discard solids. Cool completely. Pour mixture into 4 (8-oz.) jars. Cover with lids. Store in refrigerator up to 3 weeks.
TO USE THE SYRUP: Add to gin, vodka, or bourbon cocktails; iced tea; or lemonade.
PACKAGE IN: Cork Boston Glass Bottle ($2.12; specialtybottle.com)