For the salt rim, pour the juice of 1 lime and salt separately onto 2 small plates. Dip the rim of the glass first into the lime juice, then into the salt. Leave to dry.
Remove the seeds from the melon, remove the flesh from the skin and cut into large pieces, add 8 cl lime juice and puree finely. Fill the melon pulp into a shaker, add Triple Sec, tequila and vanilla syrup and mix well.
Pour the watermelon margarita into the glasses and serve.