Evenly divide the lychees cut-side down among 4 parfait glasses.
Add the gelatin powder to a large measuring cup. Add 1/2 cup of boiling water and stir until the powder is dissolved. Stir in the lychee syrup and vodka. Pour over the prepared glasses and refrigerate until gelatin sets, about 2 hours.
Meanwhile, whip the cream. In a large bowl add the heavy cream and whisk briskly until soft peaks form, then add the sugar and vanilla and continue to whisk until firm peaks form when the whisk is removed.
Once the gelatin sets, top with whipped cream and a cherry for garnish.