In a small saucepan, bring water, sugar, and vanilla bean just to a boil, stirring to dissolve sugar. Press vanilla bean in order to let caviar escape. Cover, remove from heat, and let cool.
Juice lemons, and pour into 2-quart pitcher. Add sugar mixture (including vanilla bean), then fill pitcher with cold water and stir to mix. Chill until ready to serve.
If your lemonade tastes slightly bitter, add a tiny pinch of salt to bring out the other flavors.
Notes
You can substitute 1 tablespoon vanilla bean paste or vanilla bean powder for the whole bean.