For the salt-sugar rim on the glasses, pour 1/4 cup of the lemon juice in a shallow bowl. Combine the salt and sugar on a small plate. Dip the rim of the glasses first in the lemon juice and then dip them lightly in the salt-sugar mixture. Set aside to dry.
Combine the vodka, the 1 cup of lemon juice, sugar syrup, and limoncello in a large pitcher. Just before serving, pour in the sparkling water and stir.
Fill each glass with ice. Pour the cocktail mixture over the ice, garnish each drink with a slice of lemon, and serve ice cold.
To make this sugar syrup, combine 1 cup of sugar with 1/2 cup water in a small saucepan, bring to a boil, and cook until the sugar dissolves. Cool.