Bring 1/2 cup water to a boil in a small saucepan over medium-high heat. Remove from the heat and add the gelatin, then whisk until fully dissolved. Add the sugar and whisk until fully dissolved. Let sit for 5 minutes.
Meanwhile, split each lemon in half lengthwise. Gently remove the pulp from the rinds to a small bowl, being careful not to tear the rinds; reserve the rinds. Mash the pulp until the lemon juice releases. Strain through a fine-mesh sieve and reserve 1 cup fresh juice. Discard the pulp.
Mix the lemon juice, cranberry juice and sugar-gelatin mixture in a large pitcher until combined. Add the vodka.
Place the lemon rind halves on a rimmed baking sheet and fill each with the gelatin mixture. Chill until fully set and the gelatin does not stick to your finger if you touch it, about 8 hours and up to overnight.