Pick 8 cherries for garnish, leave the stems attached, and scoop the pits from their bottoms; set aside. Stem, halve, and pit the remaining cherries and place in a heatproof container. In a medium saucepan, combine 2 cups of water, the sugar, and the vanilla seeds and pod and bring to a simmer. Pour over the top of the cherries, cool to room temperature, cover, and refrigerate for 2 days.