Sprinkle gelatin over juice concentrate in a small bowl; let stand, stirring once or twice, until the gelatin softens, about 6 minutes. Stir together sugar, corn syrup, water and salt in a 3- to 4-quart saucepan. Place over medium-high heat and cook, stirring, until the sugar dissolves. Then increase the heat to high and bring the mixture to a full boil, stirring. Stir in the gelatin mixture and vanilla and return to a boil, stirring. Boil for 1 minute more. Remove from the heat and continue stirring until the gelatin completely dissolves.