This recipe, from "Classic Liqueurs" by Cheryl Long and Heather Kibbey, needs 1/2 to 21/2 months to age. Glycerin is available in drugstores, winemaking supply shops and some herb stores. It's used to give body to thin liqueurs.
Combine rum and vanilla beans in a glass container and shake to mix. Age in a cool, dark place for 2 to 3 weeks.
After aging, In a small saucepan, combine sugar and water. Stir over medium heat until mixture comes to a boil. Remove from heat; continue stirring until all sugar is dissolved. Let cool.
Strain aged rum and vanilla bean mixture by pouring through coffee filter or cloth bag placed in strainer over medium bowl. (Save vanilla beans to make vanilla sugar; rinse and let dry completely, then place in a pint jar with 2 cups sugar and let age for a few days. Use vanilla sugar in baked goods.).
Combine cooled syrup with liqueur. Stir in glycerin. Bottle, cap and age in a cool, dark spot for an additional 1 to 2 months before serving.