In a small saucepan simmer syrup ingredients, stirring, until sugar is dissolved, about 1 minute. Remove pan from heat, add split vanilla bean, and cool syrup completely.
Discard zest and vanilla bean. Syrup may be made 1 day ahead and chilled, covered.
Just before serving, in a medium bowl combine melon balls with syrup and divide mixture among 6 martini glasses. Top each serving with a generous dollop of cream and garnish with grated zest.