Spray 2 (13x9-inch) pans with cooking spray. In large bowl, sprinkle gelatin on tonic water to soften; let stand 5 minutes. Pour 1 cup of the boiling water on gelatin; stir until gelatin is dissolved. In medium bowl, mix condensed milk and remaining 2 cups boiling water. Add condensed milk mixture, rum and vanilla to gelatin, stirring until well blended. Pour mixture evenly in pans. Refrigerate at least 6 hours or overnight until firm.