For syrup:
2/3 cup sugar
1/3 cup fresh lime juice
1/3 cup water
2 limes, zest removed in strips with a vegetable peeler
1 vanilla bean, split
4 cups 3/4 to 1-inch honeydew melon balls (from one 5-pound melon)
4 cups 3/4 to 1-inch cantaloupe melon balls (from one 5-pound melon)
Garnish: whipped cream and freshly grated lime zest
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.