Make simple syrup mixture – combine equal parts of water to sugar in a sauce pan. Warm and stir until sugar has dissolved in the water. Remove syrup from heat and let cool to room temperature.
Cut vanilla beans in half lengthwise.
Pour off ½ cup of rum from bottle and save in different container for future use.
Add cut vanilla beans to bottle.
Pour simple syrup mixture into bottle.
Let mixture sit for at least one month. The flavor will continue to get better with age.